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?
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.
The "external link" icon on desktop helps the user understand that they're leaving the website for another one; it's used because the transition is often subtle, and users might be confused by different navigation if they didn't catch that they were leaving.
In an app (at least as of iOS 15), when a user taps a link that will open another app, they'll be stopped, asked to confirm that they want to proceed, and they'll provide decision feedback to the system. Therefore, I'm not sure a visual signifier is necessary... the interaction itself is more obvious.