Do not save your custom families inside the default Autodesk folder. Why? When you uninstall Revit, the ProgramData\Autodesk\RVT 2019 folder might be deleted.
Downloading the library is only half the battle. A disorganized library leads to lost productivity and frustration. Here is how to optimize your Revit 2019 content: Autodesk Revit 2019 Library Download