Electrical – Different gauges of wire

30 amps is where a 30 amp breaker should cut off, and to avoid that, you should only use 80% of the breaker's capacity. So for a 30 amp breaker, you shouldn't be using more than 24 amps. Using a 50 amp breaker and a dedicated outlet ensures that you don't exceed the capacity of the circuit with that appliance.

Due to a special exception (NEC 210.21.B.3), you are allowed to use 15A receptacles (plural) on a 20A circuit. In order to be UL listed, all 15A receptacles must prove they can handle 20A in all respects (except 15A to any one plug, since that still means 15A).

Almost every 120V receptacle is a duplex (double). If you have a simplex (single) 120V/15A receptacle, that is special. As Ed Beal notes, since only one thing can be plugged in and it must be <=15A, the breaker must be 15A. NEC 210.21.B.1.

if you switch a hardwired load, the switch only needs to be rated for the hardwired load.

If you switch a single receptacle, the switch must be rated for the maximum of that receptacle (perhaps 15A). If one switch controls two or more receptacles, you could plug anything in there, and the switch must be rated for whatever the breaker is rated for.