This library if for microcontrollers with an onboard CAN Controller.
Esp32 and high-end STM32 have onboard CAN Controllers. For STM, it isn't always obvious if it supports CAN e.g. blackpill_f411ce doesn't support CAN but f405 often does.
The API is supposed aims to be similar to the Arduino Uno R4 (Renesas) API. There are some small areas where this is not possible (E.g. the use of pre-initialised CAN - stm sometimes uses this as a CAN_TypeDef e.g blackpill)
This aims to be a true Hardware Abstraction across multiple vendor/series.
By true, I mean that you could build Messages on this that would work across esp32, stm can2b, stm fdcan. Same API, different hardware.
The examples folder has examples of sending/receiving standard/extended frames in loopback and normal mode.
A bit ununsual but there are 'onboard' unit tests, that you can try to run.