[muzani]

I'm surprised nobody has mentioned mobile yet. While native mobile works through the Android/iOS API, it's often quite close to the minimum abstraction level.

Here's a doc you can dig around with https://source.android.com/docs/core

You get a high level overview, then it explains how everything connects right down to the hardware. It's open source too, so you can go in there and poke around.

If you want something that can make money, I'd say look at camera and Bluetooth, because these are the things that need the most customization. Neural network API could see a lot of use in the future too.

But there's plenty of fascinating stuff, like how it renders fonts, how it handles hearing aids, and so on.

Edit: TIL Android has a category of 'rich haptics', where it gives tiny haptic feedback when you swipe your finger across a surface or to the beat of music. Very few app devs know this, so it's not integrates well into apps.