U-Boot is a topic at various conferences each year. These talks might help you learn a bit about U-Boot:
Tutorial: Introduction to the Embedded Boot Loader U-boot - Behan Webster, Converse in Code from Embedded Linux Conference 2020 (slides).
Recent Advances in U-Boot - Simon Glass, Google Inc. from Embedded Linux Conference 2023.
See elinux_talks for a more comprehensive list.