The hard way. Exactly how hard depends on what tech the game is built on. Easy access to code and assets is typically lost when “building”, but you can often decompile things at the cost of losing names for things, comments or whatever. A trained eye can still tell what’s going on in the resulting mess, and make any desired changes. Some projects intentionally try to make it harder in various ways, with obfuscation and attempts at tampering detection.

A similar exception (because it can get pretty complicated) is the modding API Nemo mentions. You implement an interface (contract outlining what your code can do) and the game calls your code at what it decides is appropriate times, like updating every frame or when it’s time to draw custom stuff. This can help make sure different mods don’t instantly conflict if they try to mess with the same part of the game.