What Apple does is load all images in all emails on their server, instantly when they arrive, before you open the email or not. So the sender can't know if you saw the email and track email open rates.
I think Gmail does the same now too, I tested that site with my Google Workspace address, it got hard spam filtered (never even reached the spam folder) but it still saw 3 image loads from a google server.
Loading images through their servers and throwing off the tracking software.