I am a beginner, so if someone more advanced offers a response feel free to assume it’s based on more knoweldge than mine!
I have been learning from violintutorpro, violinlab (which I highly recommend) and a private instructor. Correct posture is sort of complicated for holding the bow and holding the violin,
but for just standing or sitting, it is good posture as in all things — shoulders back, low, relaxed. Back straight. Violin arm comes out at a 45-ish degree angle and head turns directly towards the violin. Bow arm remains relaxed and moves very little above the elbow.