I went looking for exactly this information the other day. I was surprised to find that it's hard to come up with recent, detailed explanations of what Apple is doing for telemetry collection. When they announced their DP systems back in 2017, they were clearly doing something like Google's RAPPOR [1]. But it's been several years since then and their writeups haven't been updated very much at all [2].

This is pretty important, because these systems aren't so robust that you can just assume everything is working without review. (See, for example, this paper [3].) Apple should at least document what kinds of data are being collected, and precisely how the collection process works.

[1] https://static.googleusercontent.com/media/research.google.c... [2] https://www.apple.com/privacy/docs/Differential_Privacy_Over... [3] https://arxiv.org/pdf/1709.02753