Despite having read about it multiple times I still don't feel like I know what it is. My best guess is that it's an operating system, minus the kernel? But also it can be used as a GTK/Qt type interface layer? Or it can be used to replace X/Wayland? So like, a super modular operating system? And now I guess it's also a web browser, or a generalization of web-browser-like things that may or may not actually be compatible with the traditional web?

You're close! I'd recommend checking out this blog post which frames it as such and goes into motivations and how the architecture ends up panning out at a high level:

https://arcan-fe.com/2021/09/20/arcan-as-operating-system-de...