Contain
A few things behave differently to what you might expect — usually because macOS itself allows nothing else. These are the ones you will actually notice, and the same list is in the app: Settings → Limitations, or the ? beside the version in the tab bar.
The green button fills the whole screen — title bar gone, every contained window resized to match. What it cannot do is take the menu bar with it: macOS puts a full-screen app on a desktop of its own, and no other app's window is allowed onto that desktop, so a contained window would simply be left behind. Contain therefore fills the display on the ordinary desktop, and the menu bar stays — showing the menus of whichever app you are working in. Leave with the ⤡ button in the tab bar, ⌃⌘F, or Esc.
A window in full screen sits alone on a desktop of its own, and macOS will not let another app move it while it is there. So containing one takes it out of full screen first, back to the size and place it had before. Release it, or quit Contain, and it goes straight back to full screen.
Windows on other desktops are listed too, but macOS only hands over a window on a desktop that is showing. Pick one and Contain switches over for a moment — using the app's own Window menu, the way you would — collects it, and brings you straight back. Without Screen Recording it cannot read their titles, so an app's far-away windows share one tile and you get the first of them; turn previews on and each is listed and fetched by name.
A few apps refuse to go below a size of their own — System Settings is one. Contain makes room for them when you first add them, as far as the screen allows, and centres them neatly when it cannot.
macOS can only tuck away a whole app, never a single window. So when one app has several windows contained, the ones you are not looking at sit directly behind the selected one instead of disappearing — and an app that still has a window open outside Contain is left on screen entirely, rather than taking that window away with it.
An app you hid before Contain ever saw it appears in the picker as one entry that brings it back first — then its windows are pickable as usual.
Drag the Contain window quickly across the screen and the window inside runs a fraction behind it — macOS only lets Contain move another app's window by asking, and the answer takes a moment. It catches up as soon as you slow down.
Contain works with windows that already exist — it does not launch apps for you. Open what you need, then contain it.
None of these is a setting you have to find: Contain does the sensible thing on its own and says so where you meet it. What it does do is on Features, and what it needs from your Mac is on Requirements.
€6 once · 3 Macs
Contain is at v1.2.4 — one price, no subscription, no account. Download it, paste in a key, and the windows you already have open become tabs in a single window.