The poi file is a easy to understand file.
First there is a list with all pois in the system then a per worldname poi list.
each poi added to that list is build up like :
Translated it would be:
So first there is the POI Name this is the name of the file til the _poi.yml so anything before that.
then the ylevelrange or ground
Y levelrange means a low and max y level i may spawn in. the ground option means place it on the toplayer of the world on the ground. (this goes into terrain and on terrain but will never float fully or be underground fully.
then the chancetospawn which is beside the main change of any poi spawning the chance this one gets spawned.
and then the loottables its as simple as entering the loottabe number and if u want more then 1 do it with - as seperator.