Why is Android so stupid that if you open an app B from a link inside app A then app B is somehow opened inside app A, not as an independent process?

Now you can't alternate between the two apps because there is only a single app B opened. In order to go back to app A you have to press "back" on app B many times and hope it closes and control is returned to app A.

Even if you try to reopen a new instance of app A it will just bring you back to app B that is running inside app A. It's impossible to do anything.