I just started my de-googling journey recently, and so the mechanics of notifications were still unclear to me, and I found this video super helpful.

It explains how most mobile messaging apps (including privacy-focused ones like Signal) rely on Google and Apple's centralized servers to deliver push notifications, which exposes vast amounts of user metadata.

Here's the YT link, for people who prefer it: https://youtu.be/c3ennD3wKn0