FreshStart Shelter is not an additional app sandbox. It relies entirely on the OS provided app sandbox. Shelter is an app for managing a work profile, which is a standard OS feature. Work profile management apps are considered to own the profile, not you, and they're responsible for a huge part of configuring how they work.
Profiles can be used without a management app by using user profiles. User profiles also have separate encryption keys and a much more isolated user interface.
Apps can detect the other apps installed within the same user, including across the work profile boundary to an extent. They can't detect apps only installed in other users except via exploits of vulnerabilities.
Apps can only communicate with other apps within the same profile with mutual consent between both apps. However, work profile management apps can enable limited communication between the work profile and the user profile it's nested within if they choose to do that. Work profiles are also inherently a lot less separated than user profiles.
Both work and user profiles entirely rely on the standard app sandbox and do not provide ANY additional layer of sandboxing. They provide separate workspaces for the app to communicate with from within their app sandbox, not another layer of sandboxing. They aren't a virtual machine running another OS. It's the same SystemUI process, etc.