Custom content for Sims 4 will be in package files, with the exception of Sims and Lots. So how to install Sims 4 creations depends on the type of creation.
All custom content in Package files need to be placed in the Sims 4 Mods folder. The location of this folder is usually:
Documents/Electronic Arts/The Sims 4/Mods .
Recolors show up as a variation of the original item with no specific indiciation of being Custom Content. This is for recolors of original objects. You must search for the original item and then the recolor will show as a swatch of color.
Downloaded Sims and Lots
Downloaded Sims and Lots contain each several files. All the downloaded files for that creation need to be placed in the Sims 4 Tray folder. The location of that folder is:
Documents/Electronic Arts/The Sims 4/Tray
You must unzip the file and place only the tray items in your tray, not the zip file.
Once you have placed them there, you should be able to find the Sims or Lots in your personal Gallery in the game.