In short, you cannot. UNIX systems are fully integrated, so applications become part of the system, with their executables under /usr/bin, their libraries under /usr/lib, and so on.
In terms of space however, downloaded packages go under /var/cache/pacman/pkg, and you can clean that out by removing all of the downloaded packages, or leaving only the last N versions of them — N being a number. In fact, pamac — not pacman but pamac — should do this automatically already.
Other space consumers are Snaps and FlatPaks, because they come as containers with their own sets of libraries.
That all said, if you have more than one drive and a spare partition sitting around, then take a look at this tutorial below. 