How do people know at a glance which apps have unread notifications?
Basically, you don't. At least, not in the same way as an Android phone. Notifications are visible on your lock screen or by swiping down from the top to the left of the notch. I'm an Android to iPhone switcher and app notifications in the top bar is the thing that I miss the most.
The SE v3 was reportedly a poor seller (as was the mini). Not enough fans of Touch ID and a 4 1/2 inch display to satisfy Apple.