Please forgive me if this is steps you've already taken.
It's my understanding that after you've downloaded your Sims from say TSR, you need unzip the file and copy the resulting files (similar to what's below) into your Tray folder.
For example copy these files into your Tray Folder:
0x00000000!0x00400c4a8177000e.householdbinary
0x00000001!0x00400c4a8177000e.trayitem
0x00000013!0x05400c4a8177000f.sgi
0xbe397a02!0x00400c4a8177000e.hhi
0xbe397a03!0x00400c4a8177000e.hhi
If the creator has mentioned the Sim having custom content this needs to be separately downloaded, unzipped and added to your mods folder.
At the Sims 4 main menu, click the Gallery button or Press F4. Inside the gallery, tick Include Custom Content this will help the Sim show up in the Library tab.
Click
here to learn more about downloading and Installing Sims and Lots from TSR
I'm not sure what to do if you find the files being overwritten when unzipped. You may want to look into organizing the files in your Tray Folder.
How-To: Organize the Files in the Tray Folder
https://www.carls-sims-4-guide.com/forum/index.php?topic=26535.msg442817#msg442817There may be a mod that can help organize your Tray folder, but I am not for sure.