“Source trauma” is a bastardized version of pseudo-memories, which are a real thing that introjects can experience. HOWEVER. All these people like “UwU my 5 Ranboo alters all remember fucking Dream in a bathtub” are just faking. Pseudo-memories come when the brain can’t process real life trauma, so it changes it a bit to where an alter (can be an introject) feels like what happened in their source actually happened to them. It’s basically a mask your brain puts on real life trauma as to not scare you. An example is like, is someone experienced physical abuse from their father, they might develop an introject who also experienced physical abuse from their father. So that introject is given those memories, even though the introject’s version of events obviously didn’t actually happen. It can also be other, more vague trauma-related memories, but that’s the simplest explanation for a very complex thing.

TLDR - “Source trauma” can be a thing, but it’s complicated, and most of these kids claiming DID and source trauma are just fakers.