Rooms, sims and lots are all stored in the Tray Folder. In the game, you will find these in 'My Library'.

Documents > Electronic Arts > Sims 4 > Tray

You can download rooms, sims, and lot files from other simmers to add to your game! When you download these files, they get added to your Tray folder. If the download comes with custom content, you add those files (.packages) to your mods folder. Check out my 'How to install CC' page for more information.