What was not obvious to me is whether Niri supports something like a fixed panel that would display stuff like clocks, network status, battery charge, CPU load, sound volume, etc, etc.

Those are provided by desktop shells rather than niri. For example Noctalia [0], or DankMaterialShell [1] which is build for niri specifically.

[0] https://github.com/noctalia-dev/noctalia-shell [1] https://github.com/AvengeMedia/DankMaterialShell