Basically, your assumption at the end there is correct. You cannot give outside users access to your content via field apps without the appropriate user type, which needs to be paid for.
For outside users, it's possible to use other interfaces, such as Survey123 or a mobile-friendly Web App in order to collect data.
Unfortunately, there's no way to share a map / layer to specific users outside of your org without it being generally available to the rest of the public as a whole. If they belong to some other organization, you could try creating a Collaboration, but that's about the only other way, short of adding more user types.
- Josh Carlson
Kendall County GIS