The iPhone 12 series enabled with support for 1080p video calls in FaceTime. As usual, Apple takes good care of its older devices too, so the iOS 14.2 update quietly added 1080p support for older models as well.
The 2017 generation (iPhone 8, 8 Plus and X) and later have been updated, including the budget iPhone SE (2020). Previously, they were capped at 720p for video calls.
This doesn’t quite put them on equal footing with the 12-series, which can also do 1080p over 5G – either Apple doesn’t believe 4G is fast enough to beam up FullHD or it’s a bit of product differentiation.
Anyway, note that the older iPhones have 7MP front-facing cameras while most later models have 12MP cameras (all except the new SE, really). That will affect the image quality you see, but 1080p should still be a big boost over 720p.