Recently I had to look into an issue with the Dynamics 365 for Finance and Operations mobile application, specifically I was looking at the "normal" mobile application, not the special warehousing one, so I thought I'd share what I learnt.
My initial impression coming to the mobile app, was that when I'm publishing mobile workspaces, and then running them on my mobile device, that most of the code was running on the mobile device, and that I'd have to do something fancy to see what it was doing.
That was completely wrong! All X++ logic is still on the AOS (sounds obvious now!), the mobile application is just displaying results back to the user. That means my first tip is to use TraceParser to trace what the application is doing - same as looking at an issue in the desktop browser, the trace will show X++ running, SQL queries, timings etc..
My second tip is related to the first - attach to the related AOS and debug the X++ logic behind the mobile workspace. Using TraceParser first will show which forms/classes its using so you can set your breakpoints in the right places.
The particular issue I was looking into wasn't related to X++ logic though - the problem was if I logged into the mobile application it worked fine, but if I signed out, then I couldn't log in again without uninstalling and reinstalling the app. For this issue I wanted to see how the mobile app was communicating with the outside world - with ADFS (this happened to be on-premises), with the AOS. Normally in the desktop browser I'd use Fiddler to see what calls were being made and pick up errors in communication, the good news is that you can do the same with a mobile, you just connect he device and your laptop to the same WiFi and then set the device to use Fiddler on your laptop as a proxy (as described here). This setup gives you the ability to make tests on your device and see the results immediately in Fiddler on your laptop, just like you would with the desktop browser.
It is also possible to debug the code running on the device itself, but I didn't need to do that for my issue, so saving that for a rainy day.