The 4 modules (WW, Ansible, Earthsea, and Meadowphysics) are really just small computers with different i/o configurations. Some have more knobs, some have more jacks, some have more inputs, some have more outputs. The firmware for each is completely different but some can be run on multiple modules.
You can currently run WW with the traditional WW firmware, Kria, or Orca. It will only host one firmware at a time and you must load that firmware with your computer.
Ansible runs Kria and Meadowphysics and can switch between them with a push of a button making it very versatile in general. Its the only module that can switch so fluidly at the moment. It also runs two firmwares for Arc that can be accessed also by simply connecting an Arc. Lastly, it can function as a USB host for a MIDI controller. All of this is covered in great detail in the manual. You really need to start your research there.
Earthsea only runs Earthsea. There’s no other module in the lineup that currently runs Earthsea though that could change at some point. Earthsea is unlike any of the others and cannot be duplicated or covered by any other module in the series. That being said, its function is very basic - it turns Grids into essentially a keyboard with a pattern recorder. You could achieve a similar thing with a MIDI controller and Ansible in MIDI/CV mode although without the pattern recorder (unless your MIDI keyboard has one built in).
Meadowphysics only runs Meadowphysics but there are talks of 3rd party firmwares that may or may not ever be developed for it.
Again, Ansible is by far your best bet especially if you intend to acquire an Arc at some point.
I’m not really sure what you mean by running the output of Kria into another Ansible running Kria - its certainly possible to clock another Ansible with the trigger output but you can do a lot of this within Kria itself without another sequencer involved - that’s the beauty of it. Still, Kria opens the most doors for the smallest footprint. You can connect or disconnect any note sequence from its track sequence. Lots of possibilities this way. It combines a lot of different sequencing techniques. But you could also program specific melodies, drum patterns, or even chords if you wanted.