@miori90 I am happy to help! To download from the community gallery you can open your game and in the upper right corner you will see an icon that looks like two squares/photos on top of each other. Click on that icon and you will see tabs for the gallery (news, your userID) and library. Choose gallery and you will be able to search for builds, rooms and households from the community there. Any that you want to download you should select then click the download icon and once you have don’e this they will appear in your library tab (the one I mentioned before in your game). Once a lot, room or household has been downloaded to your library it will be available off line and available to select for placement in your game.
This link should have lots of helpful information on this subject, including how to place & use the items you have downloaded: https://www.carls-sims-4-guide.com/gallery/