How to get more content in-game?
If you want tracks, you can download them from TMX "Trackmania Exchange". You said you have steam version. Yes you can download products which were published to steam workshop such as skins, tracks, and other things. If you want acces to ingame store, you have to know special manialink, which opens a ingame store. Stores are usually made by players. You can download skins, avatars, horns,tracks and mods from there. But you have to pay with your planets, which is your ingame currency.