Causes
as stated in Overview, DID is caused by extreme trauma caused at a young age, typically younger than 7 years old (the exact age varies from source to source, this is just the number i chose to use, pending better information), when your personality is first forming. it's caused from any form of extreme trauma, typically abuse, however the causes can vary.
DID is formed because going through such an extreme trauma causes your mind to think that it will not survive, and it goes into panic, or "near death experience" mode, and in most cases that causes typical trauma, but at that age, due to your personality not being fully formed, you develop DID, or multiple personalities that are separated and compartmentalized within your mind, these facets are called Alters, and each of them has their own unique memories, experiences, etc, and sometimes their own mental health disorders or issues, whether those be mimicked (for example, the system may have Autism, but an Alter may act in a way that seems to be ADHD as the disorders are similar), or the Alter be a symptom holder (the only Alter to feel those symptoms to "take the burden"), or these issues be unrelated to disorders, and just be times of sadness, depression, self harm, or any number of other things
due to the causes of DID being extreme, some Alters may be extreme as well, and some differences between said Alters may also be extreme.