I’m still very much in learning-more mode when it comes to Block Themes in WordPress. While I’ve been comfortable with “classic” themes for almost my whole time in WordPress I was waiting for WordPress 5.9 to understand block themes and how they differ. For precisely that reason I really appreciated this article (from old WPShout friend) Eric Karkovack summarizing the major points of them.
I’m happy to hear that the template hierarchy survives, just in the template directory. And after reading Eric’s article I feel pretty sure that when I finally have some free-time (and especially once I learn how to make webpages look good 
I’ll pull one note of solace from Eric those feeling a bit new to this whole topic as well:
Finally, block themes do not spell the end for classic themes – at least not for the foreseeable future. With that, the transition to this new way of theming doesn’t have to be immediate. It’s OK, for example, to experiment with blocks in a local environment while still relying on your favorite classic themes in production.