Part of the messaging (the title and buttons) is built-in, but in all of these sorts of dialogs apps provide their own strings to explain why they're asking for permission. The screenshot in the original article shows where Facebook can explain to end-users why they need to track them across other apps and websites.

In addition to my sibling comment, most apps now have pre-prompts for permissions, like where you see a full-pager telling you why the app wants to use notifications where you consent before getting the real consent prompt.