For the moment

Floating and excluding windows

Keep one window out of the layout, or leave an app alone altogether.

Float one window

Press ⌃⌥⇧F to take the focused window out of the arrangement. Press it again to put it back.

In Settings > Arrange, the Apps section lets you leave an app alone, or give it a fixed position.

In Settings > Switcher, use "Leave out an app".

Full-screen and fixed-size windows are never arranged.