I did some digging and it seems that RedM is a mod for RDR2, using the same game engine. Some players have reported success playing RedM with cracked versions of RDR2, but there's also a risk of getting banned or encountering stability issues.

The key point: RedM still relies on the core game files (maps, character models, animations, weapons, sound effects, and the game engine) to function. It does replace the game—it extends it.

Both versions are compatible as long as they are purchased through official storefronts like Rockstar Games Epic Games Quick Start Tips Install the Base Game