the constants are defined in the sketch.
They do not come from the MCP2515-library
To a certain extent this might be confusing. I have connected the MCP2515-pins to the pin-numbers on the RP2040 pico.
The names come from the Demo-Code on which my code is based on.
I guess the logic behind it is
This code runs on the pico so pin-numbers belong to the pico
and they are connected from pico to a MCP2515
hence the name MCP2515.....
best regards Stefan