I don't know reliable numbers for this and I'm not sure they exist. Would love to hear though!

But just take stock of your surroundings. Think of everyone you know, and think who of them sideload apps on a regular basis. Now correct for the tech-affinity of your social surroundings (probably high if you hang out on HN...)

For me, only the most hard-core of my friends sideload at all. Everyone else goes with stock apps. So sideloading should be in the regime of "count the zeros before the first non-zero mantissa digit."

I don't live in anything remotely close to a tech bubble and nearly everyone I know with an Android has sideloaded apps at least once, including my parents.

Don't confuse the US experience for the global one.

Thanks to China yes, they have their own ecosystem.

Lots of apps just get ripped off the android ecosystem and put on the chinese ecosystem if they're good enough.

Yeah china is different.

Instead of play store, "Default app store" to cover what ever comes with chinese phones.

Or, US and Europe, what percentage...?

Because there isn't a single unified store (vendors have their own), it's extremely common for Chinese websites to have a link on their homepage/in their header to an apk file that can self-update.

It's in the single digits or even lower. None of my non-tech friends know such a thing exists.

Do note that google engaged in anti-competitive practices to discourage other stores emerging.