It depends on your usecase. We need more detail regarding what application (gui/cui/open source/proprietary/custom) you are trying to use

• Check if the application you want to use exists as snap/flatpak/appimage.
• If it exists, install the application as per their standard operating mechansim
• Once used, just remove them. Your original system libs are untouched

If the application doesn’t exist in those platforms, it’s a different story. You might need to use distrobox or docker. Don’t use nix because it’s overkill for what you want to do.

Yes

That’s called containerization

You can use appimages, more importantly if you make a directory next to the appimage with the name of the appimage + .home the appimage will also set that as its $HOME that way you can also keep the configuration files of the app separated from the host OS.

You can also sandbox appimages with aisap.

Distrobox

There is always firejail which is very “even your mom can use it” in terms of usage.

e.g install it, type cd / && sudo firecfg, then your password and thats it.