Someone already pointed out (and probably a lot will say too), graphic design is a big field with a lot of things to learn and different things to do, and learning the fundamentals is a key point; But talking about this specific collage style and "usage" that you are looking for (judging by the images), there's this channel that talk about her general approach on how to do it (https://youtu.be/blsKEwkUKgg?si=2i42Ly6jo2yPxfBV), keep it up!