I'm not sure how mainstream of an opinion this is, but I'd suggest skipping Google's own tutorials. It would be reasonable to suppose that a multi-billion-dollar corporation that massively benefits from a thriving developer community would put out first-rate documentation, but I was quite disappointed by how much of an inconsistent and incomplete mess those tutorials are.

While I've only briefly skimmed it myself, I've heard a lot of good things about The Busy Coder's Guide to Android Development. Newer editions require a $20 "subscription", but older ones are available for free [1].

[1] https://commonsware.com/Android/4-2-free

Android development is a mess. Much of the documentation and resources you will find will be outdated. Last time I looked at the code labs from Google many where already deprecated. The Udacity courses are more current than the offical documentation. I'd recommend Programming Kotlin https://pragprog.com/book/vskotlin/programming-kotlin by Venkat Subramaniam if you don't know Kotlin. It's honestly a bad time to learn Android as the ecosystem is in flux with Jetpack and Kotlin and Android X changing everything up.

I recommend working through the beginner and advanced programming for Android tutorials on Udacity, both are free:

https://www.udacity.com/course/developing-android-apps-with-...

https://www.udacity.com/course/advanced-android-with-kotlin-...

That gives you a solid start to look into more details and do some codelabs on

https://codelabs.developers.google.com/?cat=Android

or watch a series about a certain topic on Youtube. I really like the Coding In Flow channel:

https://www.youtube.com/channel/UC_Fh8kvtkVPkeihBs42jGcA

This may be a little of topic, but I think that Google's Flutter is really game changing. It let's you write your app once and deploy down to iOS and Android and does so compiled to the target environment. Compared to other cross platform mobile development frameworks, I think they're at the top performance wise.

The Ray Wenderlich stuff is pretty good. (Full disclosure, I wrote a book for them so I am biased :))

There used to be this awesome resource called codepath. They took a developer from tutorials to actual intermediate/senior tier. It was a bay area hands-on thing but I would frequently watch their recorded videos or their online documentation. They still have some material up but a lot is gone (e.g. videos) and I am doubtful if things are up to date. But yeah .. that was awesome and I still point my engineers to what remaining resources they have left online.

As you also mention iOS I highly recommend Paul Hudson's books at https://www.hackingwithswift.com

His 100 Days of Swift (and Swift UI) are also quite good to work through. He is widely known and respected in the iOS development community and works hard to keep it all up to date (all updates are included if you purchase his books).

This is without knowing your existing background in programming, or your ambitions.

I was interested in something similar, many years ago, and realized that the app I wanted to write didn't call upon any of the internal resources of the Android phone. So I was able to write it in Javascript / HTML, and it runs on any platform in a browser.

There are some good code examples on http://guides.codepath.com/android

They're all outdated now, still in Java. It's open source, so I might actually get around to contributing to them.

Before learning to build native apps, I’d try out something that is write once, deploy anywhere like ionic, react-native, vue-native, etc.

Being able to ship to both platforms from the beginning creates a different kind of momentum.

There are some good articles/tutorials on raywenderlich.com. https://www.raywenderlich.com/android/articles

The Free Android Notes for Professionals book

Link: https://books.goalkicker.com/AndroidBook/

https://developer.android.com/ and start reading