This is a Gnome Shell extension.
It filters the windows shown in the window switcher to those of the current monitor.
You can enable it from its page on the Gnome extensions site: https://extensions.gnome.org/extension/1437/current-screen-only-for-alternate-tab/
Or, to install manually :
cd ~/.local/share/gnome-shell/extensions
git clone https://github.com/mmai/Current_screen_only_on_window_switcher.git Current_screen_only_for_Alternate_Tab@bourcereau.fr
...and reload gnome shell :
- Open the Run a Command dialog by pressing Alt+F2 on the keyboard.
- Enter just r in the box and press Enter.