How to put a tab detached from fullscreen Safari back into it Start Safari
full-screen Safari
open at least 2 tabs
click and drag one of them away from the tab bar
release left-click
it creates a new full-screen Safari instance, with that tab as the only tab


Now, how do I take that tab, and put it back into the other Safari window?
Make two tabs in one window, other free tab can now be brought in.
Window Menu > Merge all windows.