@maeve593 The location where you install the game itself has nothing to do with the location of the Mods folder and other user data. That is always located in Documents > Electronic Arts > The Sims 4. If you still don't see it, here's how to find the folder:
https://sims4.crinrict.com/eng/2018/07/how-to-find-your-user-folder/
If you want your user data on your external drive as well, you'll need to use a symbolic link, as described here:
https://crinrict.com/blog/2020/02/moving-windows-documents-folder-to-external-drive-via-symbolic-link.html
This folder can be almost anywhere, but do NOT combine it with the game's program files (what the EA App installed), as this can and will cause problems down the line.
If you need more help with this process, please list the full path to your user folder, where it's currently located, and where you want it to go.