How did you pefect all your body arcs in all your drawings do you already know who you want your pose to be or. Is it trial and error??
I know with poses I do a lot of thumbnailing which I dont always show online! I struggled with the whole ‘but what pose should they be in?’ and would end up making a lot of really quick messy doodled trying out different poses until I got one that felt right. Thumbnailing relaly helps with getting a buncha ideas out without feeling like you have to make a perfect drawing.
The inspiration for those poses came from a variety of places like life drawing classes, film, video games, cartoons, and observation from life. If I saw a neat pose I would eventually try to use it with a character somewhere.
Back to thumbnailing though, I think it was partially just drawing a lot of poses over and over and over again that eventually helped me get it down. I can look at ever pose and think back to a time where I first tried it out and it looked awful. It just took reference and lots of drawing it repeatedly for me to get it down and for it to become muscle memory.
There are still poses I struggle with to this day but its a good feeling when you actually do finally draw it and draw it right!
TL;DR: It is a mixture of finding reference, trial-error, and drawing the same thing over and over again.