[Fixed]-Define a Django model that is shared among many apps

If the models don’t belong to any app, they can certainly be declared in a separate app. I would create a ‘global’ app with models and views that you might use from other apps. Migrations would function as usual as long as you declare the new app with ‘startapp newapp’ etc.

If it’s something based on the users like in your example, consider extending the user model.