I'm very familiar with the visual cue of a link launching a new website as is used in wikipedia:
However I'm not sure of how this works on mobile apps, for example I have this situation:
What is the custom for this on mobile?
User Experience Stack Exchange is a question and answer site for user experience researchers and experts. It only takes a minute to sign up.Sign up to join this community
What I understand from your question is: What should be the behavior for "open in new tab" feature in mobile applications?
First of all, it is not best to compare a browser functionality with an app (Google Maps). You can do the same thing with a web browser in mobile as well.
Assuming a general question about multitasking, it totally depends on the implementation. For example,
Scenario 1: You click a button and the third party app is appended inside your app and control goes there. (Pressing back button may gives back the control.)
Scenario 2: You click a button and the third party app opens with some passed inputs to that and you still able to switch back to your application.
It totally depends on the app's behavior.
Hope this helps.