Comment by Macha
6 hours ago
> Do they negotiate terms separately with every copyright holder?
Yes
> Do they get access to the source code in order to preserve the games and make them fully offline-compatible?
Sometimes, sometimes their role is just packaging the game with a dosbox config too, or a developer provided drm free build
Some games get packaged with other emulators beyond dosbox as well. GOG includes and configures ScummVM for some applicable games, for instance.