Calling Mark Brown a modder is doing him a bit of a disservice. The man has made TONS of videos on his YouTube that has 1 million+ subscribers, so "content creator" is a far better title. On top of that, he has released 2 games as a solo developer, so "game developer" also fits. But modder? That's selling him way, way short.
A new Game Maker's Toolkit video examines how Rockstar made Grand Theft Auto III's Liberty City work within the PlayStation 2's 32MB memory limit by using a dynamic asset-streaming system. Through source-code analysis and a modded executable, Mark Brown shows how the game continuously loaded nearby city sectors while unloading areas behind the player, creating the illusion of a seamless open world.