At the end of the day, philosophical "label" words like "Existentialism" are made up.
Lots of Philosophers reject labels, even some who sound an awful lot like other people who like that same label and happily apply it to themselves.
This is ok! Philosophy words are sometimes helpful. Sometimes they're not. None of them are perfect or infallible. Like any merely human words, Philosophy words are limited. Use them as helpful hints for your understanding, more than absolute categories you can always be certain about.
Not everyone separates "Philosophy" into the same six branches I do—not everyone advises separating things into branches at all. That's ok. Philosophy is about trying to understand things better—it isn't an exact science (which is something I would also say about Science, incidentally!). The categories will never be perfect—so if a set of categories help you, use them! If not, find or invent different ones.
Many "-ism"s (like Rationalism or Marxism) are simply collections of multiple positions within various branches of Philosophy. Having the one word makes it easier to refer to each collection of parts together as a unit.
It's very common for Metaphysics and Epistemology to inter-relate, because "What actually exists?" and "How do I know about it?" are hard questions to separate. Therefore, a good heuristic for analyzing what a given "-ism" stands for is to ask what positions it takes on these two key branches.
Here's some (oversimplified) examples:
One of the very best ways to get started on learning about a given "-ism" (which is just a colloquial way of referring to Schools of Thought) is honestly to read the Wikipedia page for that school!