What is on the SD card and what on an SSD?

Hi there,
trying to get some sense of what is where on typical default install of umbrel on RPi4.
What stays on SDcard? how much (and how little) does it get used? Logs? Booting? Is it still just debian for raspberry? What changes to default? What changed between early umbrel (say 0.3.7) and recent one (0.5.1)? Why is it so difficult to find in some coherent documentation?

What about stuff on the SSD? what lives there? Is it persistant? What changes between early and new umbrel? Where are things like history of ssh commands if you are tweaking your node? Again, why is it so difficult to find in-depth documentation?