Have you seen the tech ref section of this forum? Lot of great info there https://www.electrical-contractor.net/forums/ubbthreads.php/ubb/postlist/Board/15/page/1
Manual transfer switches are rather simple- it's just a switch. The trickiest part is knowing how to deal with the neutral! The key concept is that the neutral must be bonded to ground once, and only once, regardless of how many sources feed the building. Alternately, you can switch the neutral in the transfer switch, and bond each of the neutrals to different points.
Smarter ones, the automatic transfer switches, also monitor voltage and frequency of the two inputs and automatically operate the switch. These have terminal boards on them with communication wires that tell generators to start and stop, and can sometimes command other breakers to open or close.
More complicated transfer switches are often built directly into switchboards, and use remote-control breakers that are opened and closed by a programmable logic controller (PLC) that monitors power and current levels and positions of breakers, and can be a lot more sophisticated, allowing generators to parallel with the utility for a no-break transition back and forth, or to automatically load shed, etc. (Was done by relays in the old days- you may still come across a few.) Again, not much I can really draw for you- it's just a switchboard with a computer doing what you or I might do by hand. Just with a whole mess of little wires.
You'll note current transformers (CTs) as loops of small-gauge wire wrapped around the cables in transfer switches and whever else current levels are measured. These essentially use the wire passing through the loop as if they were the primary coil of a transformer (albeit, just 1/2 of a loop!) and are used to measure current.