Does it interact with the general public, e.g, is it a website? /App . I’ve used Google analytics, or server logs when that’s not available though it’s been years since I’ve needed the latter.
Is there a product manager? If so they should already have this info, If not it’s pretty easy to implement.
All the caveats re, this is a broad question, I don’t know what industry, regulations etc you operate under but I bet your organization already has this info.
The understanding of the devices used by your users comes from the penetration strategy devised for your product. Once you are aware of that, then you can look for usage of devices and OS for these markets based on geographies and ensure that your web apps and native apps are compatible across devices and OS. It is advisable to prioritize a list of devices for different markets and test your product on those devices.
In this way, one can ensure that they are testing on right devices.