How do you save lots/builds to your computer?

You can use Sim 4 Tray Importer to save just specific houses. Save a lot (or even a sim or household) then open up Sims 4 Tray Importer, on the left it will show all your recent lots and households. Click the one you're interested in, and after the preview pic appears, choose the "FILES" tab above the pic. This will list all of the items in the tray folder for that specific lot (or sim). Then what I do is make a folder somewhere on my desktop and name it something descriptive so that I remember which lot it is. Then I copy those specific tray files into that folder. That way I always have that lot (or family) saved on my PC.