“open source” (capitalized or not) means that Open Source initiative agrees with the license. There are some conditions that I don’t remember.

It’s different from “source available” - open source should let you fork and reuse it.

For example there is microsoft shared code license, that’s “here is the code, but copyright is still ours, you can’t do anything with it but look”.

I have no idea what kind of license (if any) is unreal engine.