I’m curious mostly about the heel turning/gusset part of toe-up and how that works… Personally, turning the heel is my favorite part of sock knitting, seconded by the gusset pickups/decreases (knit - knit - knit - knit - weird stuff - weird stuff - weird stuff - weird stuff - BLAMO! SOCK SHAPE! - knit - knit - knit - knit).
Yellowness, the heel/gusset construction is pretty much a “cuff-down” thing (although I think I saw a pattern somewhere out there in cyberspace for a toe-up sock with gusset?).
Usually the toe-up socks use a different heel and toe construction called a “short row” heel and toe. With this method there is no heel flap, no picking up gusset stitches, and there is no need to kitchener stitch the toe.
If you learn to do short row heel & toes, you can knit a sock either way (from the toe up to cuff OR from the cuff down to toe).
In the pics below, the red sock was a toe-up sock. The blue-ish sock is a cuff down sock. Both socks use short row heels and toes. The other sock which has the familiar gusset, is a cuff-down sock.