I posted the pre extracted global C20 and C21 boot images, with and without magisk pre-installed in previous posts.
You don't need to do it yourself unless you want to learn for the next update (or if you are in a different region).
I figure I'll keep posting the boot images whenever I extract them, it's a pain to do, and I wouldn't expect most people to want to deal with it.
Edit: For people in other regions, sorry, I don't have the files on hand. If you provide links to a full OTA, and the associated incremental OTAs, I'll gladly extract the boot.img for you. Just post the links.