It is considered one of the best methods to load custom maps on PC. How to Set Up Custom Maps (Step-by-Step)
It allows for storing assets inside your level package, essential for functioning with the Rocket League Workshop. Labs_Underpass_P.upk
Create a backup of the original Labs_Underpass_P.upk to avoid losing the default map. It is considered one of the best methods
Find your Rocket League map folder, usually located here: Documents\My Games\RocketLeague\TAGame\CookedPCConsole\mods Labs_Underpass_P.upk