A mod browser would be where you can purchuse loco reskins and extra locos and mods made by the community without leaving the game, the installed mods would have tick boxes next to them to enable players to activate and deactivate mods to have access to the mod browser you would have to have a DTG account
There is one for mods, and another just for custom liveries, and they're both free. TSW 2 Mod Manager TSW2-Livery-Manager A few places you can find mods are: Train Sim Community Railsim.de Trainsim.cz Here are the tutorials: Mod Manager Livery Manager