Is loopyV updated with loopyT after every scanline?
No, only the X-scrolling related bits are.
thanks for your quick reply!
so only bits D4-D0 of loopyT are copied into loopyV after every scanline?
At pixel #257 of the scanline, if background and sprites are enabled, these bits get copied:
v:.....x.....xxxxx=t:.....x.....xxxxx
So it copies the 5 low bits, and the X nametable bit. That's how it snaps back to the left side of the screen after it has drawn a scanline.