Through the years I've gathered a few bits and pieces about "thought forms" during all my research ... as interpreted through my experiences and guidance (so take it or leave it).
I've read that people who believe in magic consider thought forms to be something of an art form, and talk about using them to manipulate and control others (which I think is only possible with "victims" who don't have internal defences or a clear understanding of their own autonomy and self respect).
From what I understand, a thought form is a fragment of reality (but not a spiritual entity) that isn't currently physical, but that someone either wants to bring into existence or is inadvertently creating out of their beliefs and repetitive imagination. This means it is either something that is in progress of coming into being through one's choices or attitudes or something that someone else is bringing/has brought into a structured mental form through their imagination, choices, beliefs, or attitudes.
There is a theory that this explains some ghosts. (Basically that they are the strong emotions or incomplete desires someone left tied to a location or experience when they died.)
People attract what they internally resonate with (consciously or unconsciously look for), whether thought forms, entities, experiences, ideas, etc. People who are psychically/emotionally sensitive in general will be sensitive primarily to anything they look for or expect to find (and one form of openness is fear, the expectation that something bad is likely to happen). People who are fearful (secretly or openly) will resonate with and be more sensitive to things that form around fear or create additional fear ... for them. This could be anything from their own imaginations consolidating around secret (or past life induced) fear and possibly visualizing it enough that a structured thought form grows from it (which would make it feel even more solid and fearsome, even though it's still an idea).
Combine this with the general truth that what people look for in life they are more likely to find, and there will be confirmation bias of "evil attacking" since any disruption to life will be interpreted through that lens. That cycle leads to greater fear, less belief in one's own freedom to choose the good in life and own one's space, etc.
So, let's say the thought form that comes to mind when they are in fear looks like a demon. It's (probably) not a literal demon. (I think there are non-imaginary spiritual entities who latch onto people who are unconsciously resonating with them, and that is one reason I strongly encourage people to remain in alignment with their integrity. Inauthenticity and self-destructive habits create vulnerability to trickery, abuse, and manipulation.)
... But back to the purely mental projection of a thought form"demon". Someone who has a significant amount of fear built up around the idea of demons is more likely to find people in their lives who behave "in a demonic way" and who basically disrespect them as people and attempt to limit their autonomy. They are also vulnerable to the fears of others. Put a bunch of people who are afraid of unseen evil in a room together and they'll begin seeing each other's demons (and adding to their own collection) because they are already vulnerable to that perception. Someone who "curses" them will get through, because they are open to being cursed. Someone who claims God has rejected/cursed them will get through, because they already believe they are unprotected. The vulnerability started with the fear, but it won't seem that way once it is strong enough to create a spiral of confirmation.
Those have no sense of ownership of their mental, emotional, psychic, and physical space fear that anything and everything can break in and harm them at any time, and that fear takes down their defenses -- mental, emotional, psychic, and spiritual -- that would otherwise keep everything out. This is why love is the answer to fear. Loving yourself allows you to defend yourself, own your space, recognize your boundaries, and hold out against both external disrespect and internal attack from one's own imagination or the ideas of others. Counselling often helps, because once someone learns to take ownership of their choices they often find their way back to stability and learn they can increasingly control the effect of these influences.
Sidenote: People who have enough connection and empathy can share their mental images with others. (My friend and I do this in a positive way all the time, often as elements in our dreams. Like one of us will dream of saving a child from drowning and the other will have a completely different dream, but will also save a child from drowning.) Sometimes this even works with strangers. (I'm of the opinion that this tends to happen more when there's a past life connection that aligns the two with whatever is being shared. It's not an "anything goes" type of connection.)
So to take this into a more positive area, I've helped friends (some with schizophrenia, etc.) use the theoretic structure of thought forms to actually create defenses, protections, and reassurance. Of course, I rarely use the term "thought form". I just say, "If it's in your head, then you can consciously influence it, too." How this works is that you recognize that even if your mind that is capable of scrambling the link between imagination, spiritual perception, and reality ... that means it can also be consciously tricked into playing into your chosen scenarios, too. It takes time and effort, but you can create (by constantly reviewing and improving) an imaginary defense that will activate against things that are perceived as dangerous and fearful, and you can take charge of the playing field in which these interactions take place. Basically, because it's your mind (psychic space, imagination, emotions) you can change the game to one you can win. Creating stable visualizations may not clear up the physical problem, but it does return some control and provide a tool and a path forward in the midst of the confusion ... and that is reassuring and stabilizing.
Of course, this has the added benefit of allowing your spiritual support team/guides/love and light/god to reinforce those defenses and help protect you, too. They can't force us against our free will to believe whatever we choose, to prioritize respect, love, and light or live in alignment with our integrity, but the moment we take the first step, that gives them a place to add their own energy and make an even greater difference.
A thought form can be something as simple as the place you always go when you meditate. When you visualise the same place every time it can become its own identity in your mind and begins to take on a stable expression that seems almost tactile and real (given that we experience everything through this part of the mind, that's not surprising).
If you create walls of white light that block evil, then however strong you make them (meaning you trust and believe in them, and act in alignment with their validity for both yourself and others), that is how much they will keep out fear (and therefore darkness and evil). Not external circumstances, but the internal story that comes along on its own or alongside outside influences. If you always imagine a being in your mind to the point where it seems to take on its own personality, you get to adjust its personality into one that isn't capable of causing you harm or if you feel threatened, you can always kick it out of your mental( and spiritual) space until it learns to abide by the rules of your inner space, the first of which is respecting and protecting you.
Anyone who isn't in the habit of seeing their mind as a "my game, my rules" kind of space won't realize they can change the playing field or evict ideas that don't play nice. We choose what and who we allow close, and what we imagine can influence our reality. I'm careful what I imagine because of this. If I disrespect people or imagine them doing what they would never consent to doing, I mentally open myself to trickery and deception. If I want to be respected spiritually, socially, or in my imagination, I start with the belief that everyone deserves that respect and hold that space as a buffer to my expectations in daily interactions between myself and others. This is an active creation of the love that counters fear. Those who don't believe in respecting me (or others) don't get close enough to intrude on my space because I set boundaries between myself and them, whether in real life or psychically.
Take it for what it's worth. Maybe some of this will be useful to you. As always, test it for truth and discard what doesn't work for you.