Chromebooks seem simple but under the hood there’s quite a bit of code that makes it all work. https://chromium.googlesource.com/chromiumos/docs/+/HEAD/crostini_developer_guide.md has an excellent fairly high level diagram of Chrome OS (CrOS). The Termina VM (right, Terminal without the trailing L) is where your “standard” Linux container (known as “penguin”) is started which gives you your full blown Debian environment. But note that at its very heart is a Debian container which starts up everything else. The reference above talks about how to MAKE CrOS.
If you want to learn more about how to USE CrOS and maybe launch your own containers instead of or in addition to penguin, read https://chromium.googlesource.com/chromiumos/docs/+/HEAD/containers_and_vms.md which talks about the mechanics of things
https://www.reddit.com/r/Crostini/comments/squfcz/trying_to_understand_what_chrome_oscrostini/ is the source article for this.