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.
If the Sims/Lots contains custom content please make sure to enable *include custom content* at the gallery to show up the creations.