In reply to A confused reader. I generally try to keep the posts readable for a more general audience here, but I am always happy to discuss the technical details further. Feel free to ask me any question about the process. For this theme, I started with the theme.json file and filled out some of the settings that I knew I would want enabled/disabled. Carolina Nymark's lesson on creating this file is a great starting point. From there, I added a templates/index.html template with a Post Content block just to keep things simple.

Since I knew it was just going to be a one-page theme with specific patterns, I could build those patterns out in the editor. All I needed was that block in the file to kick-off the process. Then, I headed over to the editor and designed the first profile-card pattern. Once I liked it, I created a pattern category via register_block_pattern_category() and pattern via register_block_pattern(). I repeated this same process for more patterns. Once I had my initial pattern in place, I also adjusted my template to display it instead of post content:

As I continued building the theme, I made adjustments to the styles key under theme.json. Anything I could add there, went into that file. If I had other style adjustments not possible with it, they went into my style.css. That is how I got from Point A to Point B. I have since changed things a bit, cleaned up the code, and made things more to my liking. I've also added features like per-block styles (these get inlined automatically). I've added a few custom block styles and such. But, the primary process is the above.