STM32
The original MicroPython port. USB CDC REPL, DFU or UF2-style loaders depending on the board, and a dense machine surface. PWM is on STM32 from 1.28; so is the common machine.CAN API. Docs: pyboard quickref.
Families
F4 is the pyboard classic. F7 / H7 have more RAM. U5 (from 1.27) is the low-power line — Nucleo-U5A5ZJ-Q has a board profile. WeAct BlackPill boards are the usual cheap F4.
Legacy pyb
The pyboard still ships pyb.LED, pyb.Switch and friends. New code should prefer machine so it ports to Pico and ESP32.