> So, I've seen signal masts with a low cabinette, single door and a signal mast atop a duel cabinette,

These are probably single and double style B cases. The searchlight mechanism was a drop-in replacement for the semaphore mechanism.

> and am asumming this is where the track wiring is going into, from where exactly?

From the track to the track relay or the track battery.

> Power cables from a lineside power pole? Or underground cables?

Most of the wires you see are line control wires. Underground was used too.

> I've also seen masts just sitting on a litle base, with a signal cabinette elsewhere with the single round post out the top with a wire going to a presumably power line between poles?

Probably what is called a "cut section", which consists of track batteries, track relays, and line control relays for two continguous track circuits between signal locations.