pico-sdk/src/rp2_common/README.md

This directory contains libraries specifically targeting the RP2040 or possible future related devices. It is selected when PICO_PLATFORM=rp2040 (the default) is specified for the build

hardware_ libraries exist for individual hardware components to provide a simple API providing a thin abstraction hiding the details of accessing the hardware registers directly.

pico_ provides higher level functionality you might generally find in say an OS kernel, as well as runtime support familiar to most C programmers.