Could you share a link?
If you're running Firefox, you can go to `about:telemetry` and see what data is there. Note that some of that data might be populated even if you have telemetry turned off. Don't reach for your pitchfork quite yet: I assure you that the data isn't being sent.
You can go to https://telemetry.mozilla.org to see various analyses involving telemetry data.
In the source tree, telemetry probes must be defined in one of a few places. Here are their locations:
https://searchfox.org/mozilla-central/source/toolkit/compone...
https://searchfox.org/mozilla-central/source/toolkit/compone...
https://searchfox.org/mozilla-central/source/toolkit/compone...
https://searchfox.org/mozilla-central/source/toolkit/compone...
> Don't reach for your pitchfork quite yet
No need to assume people are out of their minds or even critical. I am curious about how it's done on a technical level, with the old local ad system in mind (which I thought was a brilliant solution to Internet commerce and privacy). I've supported and contributed to Mozilla since before Firefox.
As for high level docs about how it works, I haven't been involved in quite a few years, so I'm not 100% sure about the best source, but this link looks like a good place to start: