NetBSD, from their own website:

The NetBSD Project's goals

A project has no point if it doesn't have goals. Thankfully, the NetBSD Project has enough goals to keep it busy for quite some time. Generally speaking, the NetBSD Project:

provides a well designed, stable, and fast BSD system,
avoids encumbering licenses,
provides a portable system, which runs on many hardware platforms,
interoperates well with other systems,
conforms to open systems standards as much as is practical.

In summary: The NetBSD Project provides a freely available and redistributable system that professionals, hobbyists, and researchers can use in whatever manner they wish.

Based on the name of have assumed it’s be used in things like network appliances but in 20 years I’ve never seen a single device use it.

The name comes from being develop over the internet, when that was still a pretty new concept. It's pretty popular among Japanese ISP's iirc.

If you're at all interested in unix, you should try NetBSD. Open has security as a focus...although some of that is overstated imo. FreeBSD is clearly targeting servers, even if it is all purpose.

NetBSD is less popular, but it's clean, lightweight, portable, has pkgsrc. Think of Net as a cross between Open and Free.