Git is the underlying code management and version control system. It can be used directly, and also forms the backend to a number of other systems.

Code “forges” are platforms which integrate a version control system (like git), a code repository (a file server), and front end utilities.

Some git forges are open source, others are proprietary. Certainly with the open source ones, but also with the proprietary ones in some cases, you can either self-host or use a hosted service.

GitHub is a proprietary forge, and GitHub.com is the company’s fully hosted service. They’re now owned by Microsoft.

Gitlab is an open source forge. Gitlab.com offers a hosted service, but many projects self-host.

Forgejo is a fork of Gitea which is a fork of Gogs. These are all also open source. As far as I know, neither Forgejo nor Gogs offer a hosted version, but Gitea does.

A few other notable forges include GNU Savannah (open source), Bitbucket (proprietary), Sourceforge (proprietary), Launchpad (open source), Allura (open source).

At the end of the day, they all do the same thing. They have different feature lists (especially around some of the project management and user interaction side), different user interfaces (some are shinier and more modern, others more minimalist), and different communities and support models. You choose that one that works best for your needs.

GitHub is probably the most feature-rich (and/or bloated) of them. GitLab is competing in the same space, and self-hosted GitLab seems to be something of a sweet spot for many projects that want a premium experience without needing to use a proprietary Microsoft product. I don’t have much experience with Forgejo or Gitea. The rest tend to exist in their niches.

There are already several Rust Kennel From Scratch projects that are reasonably progressed. Redox is one, Asterinas is another.

The latter is I think aiming for Linux ABI compatibility.

where [it] comes from

You imply it comes from:

The “thin blue line” symbol has been used by the “Blue Lives Matter” movement, which emerged in 2014

But you link to a Wikipedia article that says:

New York police commissioner Richard Enright used the phrase in 1922. In the 1950s, Los Angeles Police Chief Bill Parker often used the term in speeches, and he also lent the phrase to the department-produced television show The Thin Blue Line. Parker used the term “thin blue line” to further reinforce the role of the LAPD. As Parker explained, the thin blue line, representing the LAPD, was the barrier between law and order and social and civil anarchy.

The Oxford English Dictionary records its use in 1962 by The Sunday Times referring to police presence at an anti-nuclear demonstration. The phrase is also documented in a 1965 pamphlet by the Massachusetts government, referring to its state police force, and in even earlier police reports of the NYPD. By the early 1970s, the term had spread to police departments across the United States. Author and police officer Joseph Wambaugh helped to further popularize the phrase with his police novels throughout the 1970s and 1980s.

The term was used for the title of Errol Morris’s 1988 documentary film The Thin Blue Line about the murder of the Dallas Police officer Robert W. Wood.

I have no idea about this guy’s politics, but it’s a pretty well known phrase with a lot of different contexts.