As far as I can tell, storage scopes is an app-specific setting. It can usually be enabled when an app first asks for storage permissions. You'll receive a prompt with three options (paraphrasing from memory):
- Yes (Allow in settings)
- Storage scopes
If you choose the last option, you'll be taken to the storage scopes menu, where you can add individual files or folders that you wish to grant access to for the app.
Storage scopes can also be accessed directly from the Android Settings menu for some apps. For example:
Settings > Apps > All n apps > Syncthing > Storage scopes