Modders post on their own sites or host sites like Mod The Sims. Find the mod you’re interested in. Make sure it’s been updated. Download from the site. You may have to unzip the file before installation. To install, assuming you’re on PC, go to Documents>Electronic Arts>Sims 4>Mods and place the file in that folder. Start the game and make sure mods and cc are enabled in game settings. If not, enable and restart. That’s it. Be aware that mods can conflict with each other so you will have to check for that at times. They can also break when game updates are done, and will have to be updated by the modder and redownloaded. Because I don’t want to spend too much time maintaining them, I only play with mods and cc I absolutely have to have in my game.