This is a high level, non-exhaustive overview on how to migrate your mod from 1.20.5/6 to 1.21. This does not look at any specific mod loader, just the changes to the vanilla classes.
This primer is licensed under the Creative Commons Attribution 4.0 International, so feel free to use it as a reference and leave a link so that other readers can consume the primer.
If there's any incorrect or missing information, please leave a comment below. Thanks!