This app was mentioned in 2 comments, with an average of 1.50 upvotes
It depends on the game. Adding Play Store links to my games so you can view the screenshots.
The artwork in Rocket Mail was designed for 1080x1920 (a 9:16 ratio), stretched to fill as much of the screen as possible, and just uses black bars outside that. That would be a simple FitViewport
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Patchy only uses vector art consisting of OpenGL-drawn lines and shader effects, and emulates a low-resolution screen, so there's never any worry about the actual resolution. The playing field has a fixed aspect ratio and is stretched to fit as much of the screen as possible, keeping some margin for the text. The text on the outside is aligned to the edges of the screen, not to the playing field.
Twistago is even more adaptive. As you can see, the actual game board is square, designed at 1080x1080. The menu buttons go outside that square, extending it to 1080x1350 in portrait mode as you can see in the screenshots. But if you turn your device to landscape, these buttons move to the side, giving a rectangle of 1350x1080 instead. The background gradient was designed for the inner 1080x1080 square, and is extended with solid blue towards the outside. The animated stars do go into that blue area, so it never feels like we're adding black bars, even though that area of the screen is entirely cosmetic.
Generally, I find that the 16:9 ratio comes close to the median aspect ratio of actual devices, so it results in as little "waste" as possible even on wider or taller devices.
Rocket Mail was inspired by the strange curved routes that airplanes seem to take on a map. In Rocket Mail you use your phone or tablet to aim a parcel delivery rocket at some city in the world. It requires spatial insight, but of course also geographical knowledge!
While challenging in Normal mode, Rocket Mail comes with an Easy mode mostly targeted at younger players, possibly for playing together with a parent or teacher. This could be a great educational game.
Having made games in my spare time for many years, I recently took the plunge to quit my day job, and today is my first day as a fulltime indie game developer. I'm equal parts excited and terrified. I'm a decent enough coder, but I'm still learning the ropes of graphics, sound effects, and above all marketing. I see Rocket Mail as the start of this road: I intend to make similarly small games for a while, to get experience so that I can tackle something bigger later on.
I have a dev blog that I update often (but irregularly), and I'm also on Twitter as @frozenfractal.
Feedback about the game is very welcome! I don't intend to change it much anymore, but I'll take any lessons to heart for subsequent games.