It’s this real? What’s the mechanism by which they work?
Usually they have a link to a single pixel image (or similar) that is loaded from a remote server with a URL and/or URL param customized to you. When you open the email, the pixel is loaded, informing the server that you opened the email. However, Apple last year implemented a change in email such that it would auto-trigger the “open” for all emails, purposefully, to flood those types of trackers and make the data useless. That has worked remarkably well.
Doesn’t just having images not auto load break all those things anyway?
Maybe, but not all people have that turned on. This was more of a deliberate middle finger to tracking, on behalf of Apple. They have a history of doing it, with the IDFA, device ID, etc.