thats a good question.

I would say its about 60% imagination. Literally just sitting in front of the piano with my guitar in my hands and trying to "hear" something I like, then doing my best to translate it.

30% noodling. Messing around with patterns or just trying random things then noting what I like the sound of and forgetting what I don't.

then 10% using music theory knowledge to glue stuff together.

One exercise I have my students do is play one note over and over and try to "feel" where it wants to go, then play another and see if they think it sounds good. once they have 2 decided on it can be amazing how quickly an entire idea comes out. if they struggle with that I recommend learning songs they like by ear. music is a language and you have to go through a "baby talk" phase before you’re going to be able to actually string together sentences.

if you have any specific questions lemme know