MY experience was both ends have their own clocks that are similar in frequency. The receiving end uses a phase locked loop of sorts to adjust it's clock so the two leading message bytes, sync characters, are read properly, The second character must be read properly or the message must be rejected. Once the clocking is set, all the rest of the message is properly read and decoded.
There is a method of data encoding that can be used to include synchronizing information, but it was so rare that I never encountered it. Was never used with standard data modems. Seemed to be a lab curiosity.