Attempting to adjust height of quill stem but it won’t budge. Is it just seized or is there something I’m missing?

After you loosen the quill bolt, you need to ‘tap’ the bolt so the wedge drops. If that’s done and it still doesn’t move you can try to spray penetrating oil between the stem and fork steer tube.

If that doesn’t work, it could be seized like how seatposts get stuck in a frame. Usually stems are easier to remove because you can get better leverage on the pieces to rotate.

Good luck.