how does "remote control dimmer" work?

The dimmer itself uses a triac to control the a.c. It "lets bits of it through". The more it lets through the brighter the light. Details of that may be found in many places on the internet.

A remote controlled one works in a similar way to a remotely controlled t.v. A handheld transmitter sends infra-red signals to it, telling it what to do. A corresponding infra-red receiver in the dimmer (or t.v.) makes adjustments accordingly.