First, my strategy.
I am using a cross-platform mobile development strategy which means that rather than learn java to program Android, Swift to program iOS and C# to program Windows, I can focus all my energy on learning one programming language and using a cross-platform development tool to create apps for all those mobile platforms. And my cross-platform development tool of choice is Xamarin. It is the one with the most native implementation and most robust of all the ones available. And since the recent Microsoft acquisition of the company behind Xamarin they have made Xamarin free to use and a lot more robust. It used to cost over $1,000/year to use. Now you can get it for free at https://www.xamarin.com/download
Since using Xamarin is my strategy, then the resources that will deliver the most value for my limited time are ones around using it expertly. Before now I had taken courses on Android app development using Java (even did some demo apps), iOS app development using Swift (had to use a VM installation of MacOS on my laptop and didn't complete the course) and Windows mobile app development using C# (even successfully published two apps to the Windows app store). Now I am focusing on just learning Xamarin.
The number one resource you should get is the very detailed super useful book Microsoft has subsidized (made free) for Xamarin users: Creating Mobile Apps with Xamarin.Forms
Then you should head to Xamarin Developer and Microsoft Virtual Academy
If you can spare a few bucks then you definitely should checkout Xamarin Forms: Build Native Cross-platform Apps with C# Udemy course.