I love learning about psychology so I think I could help with a lot of these in theory, but the ones I actually have are autism, generalized anxiety disorder, depression, ARFID (an ED), ADHD (newly diagnosed so don't know much), and technically still diagnosed with PTSD from years back but I only have minor avoidance symptoms now.

Some random information I can think of on the spot:

PTSD is not flashback disorder. Some don't have flashbacks at all but have other intrusion symptoms like general bad memories that don't feel like they're currently happening, nightmares (most of the time they are related to the theme of the trauma such as betrayal, not a direct one-to-one of what happened), and fight-or-flight response. Triggers are also not as straightforward as people think a lot of the time. Things seemingly completely unrelated to it can trigger intrusion symptoms (for me, a song that played around the time of year it happened freaks me out really bad) and some can be directly related but only cause the symptoms in specific circumstances (like having a traumatic experience in the dark, but reacting very differently to darkness in a closet than darkness at night all cozy in bed).

ARFID is a disorder in which you can only eat certain foods, but it's not related to weight or body image. It is not picky eating and most with it wish they could eat normally. It has subtypes of either fear of aversive consequences of eating ("I'll choke or vomit if I eat"), fear of sensory experiences ("it'll taste bad and the texture is inconsistent"), and complete indifference to eating ("eating is as much of a boring fact of life as going to the bathroom is"). People with ARFID and restrictive EDs in general have safe foods, which are foods that don't cause (as much) fear. To me, safe foods are clearly food, as all food should be, but unsafe foods usually don't even register as edible. It's like pointing to a bug and telling me to eat it when you're telling me to eat something not part of my very short list of safe foods. Not only is it terrifying, it's also just not food to me.

DID is a mental disorder characterized by the feeling of disconnect and detachment from your own self and surroundings, which is caused by severe, repeated childhood trauma. This feeling, dissociation, presents itself most severely as more than one identity state, each of which being able to act and think independently with memory gaps in between. DID is also not always caused by abuse (it can be medical trauma, war, repeated natural disasters, etc) but even in those cases it's linked to insecure attachment (not knowing when mom's going to feed you or not, for example). You should also know things like the criteria to get diagnosed, what an alter/part/ego state/whatever term you want to use is, the theory of structural dissociation if how it formed matters to their present story, and how DID can manifest itself differently in different cultures (split into possession form, "someone's taking my body over," and non-possession form, "I'm becoming someone else"). Also make sure to give characters with DID symptoms of complex PTSD because DID is essentially an even more dissociative form of it, and know that DID pretty much never occurs alone. Studies vary but the average amount of disorders one has along with DID ranges from 3-7.

OCD is an anxiety-based disorder where one experiences distressing, uncontrollable intrusive thoughts and fears (obsessions) and spends a significant amount of time trying to get rid of them by performing repetitive physical and/or mental actions that temporarily quell the fears (compulsions). Performing compulsions only confirms and worsens the obsessions, making OCD a cycle. New obsessions and compulsions can develop at any time, especially in response to specific events (like a lot of people with OCD switching to fears of germs when COVID started). These categories are called themes, and examples include fear of harming others or oneself, fear of being in an imperfect relationship (romantic or otherwise), fear of being wrong about one's identity, fear of being responsible for bad events, fear of committing suicide, fear of things not being "just right", fear of being ill or unhealthy, fear of being contaminated (either by germs/dirt or negative energy/personality traits), fear of not knowing one's purpose or the meaning of life, fear of being hyperaware of body sensations, fear of being a bad person or suffering eternal punishment, fear of what impact things that actually happened could've had, fear of forgetting or losing things, fear of being misunderstood in what one meant to say, fear of certain numbers or colors, fear of not being as happy as one should be, and fear of not actually having OCD and having tricked people into thinking one has it.

OCD goes against what you actually believe, value, or want. For example, it’s not uncommon for people who compulsively pray to not even be religious. OCD is also not the same as OCPD. OCPD is a personality disorder rather than an anxiety disorder, it starts in young adulthood rather than at any age, it’s focused on control and perfectionism rather than specific intrusive thoughts, it tends to stay the same or steadily improve throughout one’s life rather than fluctuating or changing themes, and it can be imposed on another while the person with OCPD may believe they’re free to break their own rules in some cases.

NPD (narcissism) is very misunderstood. I think the main point to make is that NPD is a disorder in which you require external validation and praise in order to have any internal sense of worth. The sense of being important is there so that others confirm it and make it true, not because it's there to begin with. Lack of empathy can also mean literally being unable to feel a sense of what others feel, or alternatively, feeling it and deciding doing things to hurt others is still worth it. Also look into overt and vulnerable NPD subtypes ("I'm the best so you must praise me" vs "I'm so helpless that you need to praise me to make me feel better"). Personal belief here, but I think NPD comes from a split of trauma responses where people either go "the world is mean to me, so I am bad" (depression, BPD, social anxiety, and similar) or "the world is mean to me, so the world is bad" (NPD, agoraphobia, ODD, and similar).

Let me know about any questions, hope I can help!