There is still a bunch of undocumented stuff - and incomplete pages, this is what i know so far so pardon me :)
There is code you can run on the UDOS repository. See README instructions for running it.
// Enable tracing for DASD t+1b9 // Enable instruction tracing t+ // Boot the DASD ipl 1b9
Notice that Hercules has a DIAG 8 call to print debug statments, you can use it to issue commands to the Hercules console
These are documents which describes the architecture of different S390 versions (there is the S360, the dinosaur of dinosaurs, the Z/ARCH, the most modern mainframe) and so on, S360 being the oldest, followed by the S370, S390 and finally Z/ARCH as the newest shiny architecture.
Note that S390 is being used in this page as a global "catch-all" for both z/Arch and S3X0 series. z/Architecture runs the operating system on LPARs while the 3X0 series runs it on baremetal.
In addition, the S3X0 series in itself is pretty different, for example the S360 uses SIO/TIO instructions and lacks dynamic address translation, while future models (S370 and S390) do have DAT and use a different subchannel method.