I used to only rarely look at Youtube (repairs, cooking, the odd live concert) but since I started looking at it more often (for guitar and piano tutorials), I’ve found its UI to be too distracting. I added the following custom uBlock filters to make it less annoying:

    ! Remove the "Up next" sidebar (the #secondary container) so that the main content area (#primary) takes up the full width.
    youtube.com##ytd-watch-flexy #secondary:remove()

    ! Ensure the video takes up the full width when playing full screen.
    youtube.com###panels-full-bleed-container:remove()