Reservists concentrated in trawlers and Flower-class escorts

Were new sailors and reservists assigned by civilian background, such as fishermen to destroyers and farmers to Flowers?

Not in that simple way. Regulars concentrated in complex ships, while reservists and wartime entrants made up more of the crews in trawlers, Flowers, and other small escorts.

Wartime assignment was not a literal occupational sorting system by ship class. Regular sailors were more likely to fill the larger and technically complex ships that required established naval specialists. Reservists formed a heavier share of crews in armed trawlers, Flower-class corvettes, and related escort vessels. War-service-only personnel then filled gaps around those experienced cores as the fleet expanded.