There is one country where Chrome is not the most popular

Nigeria, Opera

Tecno phones come preinstalled with the mobile browser

Opera Mini was big, depending on where you were. If that brand recognition isn't why it's installed it's definitely part of why it stays. So the story starts earlier than described.

This forum used to be pretty interesting back when the mobile OS market was more diverse https://www.nairaland.com/phones

That makes me wonder: How is usage data gathered for let's say North Korea?

They have an IP space you could track visitors from. This blog post outlines it a bit: https://nknetobserver.github.io/