The Profiles folder is located in
<Documents>\Rockstar Games\GTA V\Profiles (most people say that is in
%USERPROFILE%\Documents\Rockstar Games\GTA V\Profiles, but Windows allows users to change the location of the Documents folder so this is not always true).
To install a save, you can drop it into one of the folders inside of Profiles with alphanumeric characters like "38AC7DF1". If you have more than one, try to guess which one it is based on the number of existing saves and timestamps.
If you don't have a Profiles folder in
<Documents>\Rockstar Games\GTA V, you are probably running a pirate copy. All of the versions of GTA V (R* Warehouse, RGL, Steam and EGS) create the Profiles folder to store saved data.