Can somebody explain to me how the new "rich rocks" work? Patch notes say "This ore replaces Deep Mining Drills." but still I can only get it from core fragments?

Ok, I found out that rich ore is actually a patch on the ground.
I followed this thread to regenerate resource fields: https://mods.factorio.com/mod/Krastorio/discussion/5d70aaf2420f2e000c0e1f06
Unfortunately Deep Mining Drill is still activated in the production menu and Core Fragments under intermediate products even if the box is unchecked.

Most likely the drill is available to you because you have already unlocked this technology. Since the deep drills are not completely removed, but simply hidden for research.
This is necessary so as not to break the game for those people who are already playing with it.