I only tried one (rill) and it also required a separate config file before I could start spawning windows. Anyways I think this river project is mainly interesting for two reasons:

• Allowing single developers to actually make their own wayland window manager. Possible without it, but not necessarily feasible.
• Making it possible to run a functional wayland desktop under memory constraints. River seems to use something like 100-150MB on my system.

The second point appeals to me, because my daily driver laptop is nearly 15 years old, so every MB of memory I can save is of tangible value. I’ve gone back to X11 on it because of video driver issues on wayland, but I keep an eye out for projects like these.

I just tried it for a bit, and you basically have to create an executable bash script at $XDG_CONFIG_HOME/river/init that launches your selected window manager ($XDG_CONFIG_HOME is usually ~/.config). It will not work unless marked executable, and you do that by running chmod +x on it.