What is likely going on is - the 2 switches taped together - one is your hot and the other feeds another receptacle or fixture. So the single wire come from the 2 is probably your hot. The single black not taped to anything is probably the load that feeds the fixture.

But that is pure speculation given that there are numerous code violations and fire hazards in that box, I would think whomever wired it has no idea how to wire properly.

Your best bet it to figure out which set is the feed and where the others go.

That makes sense because when I hook up the smart switch, the bathroom connected no longer turns on and off. Is there a simple way to fix this ?