Generally people start flying in airline jobs in Europe w/ a ME Commercial License and the numerous written tests for ATPL completed - at about 250 hours. Add in a couple "work like a crew" courses and this combo is colloquially referred to as a "frozen ATPL." Though it's really just multi-commercial.
The ATPL written tests require hundreds of hours of mandatory ground school, though as an existing ICAO-compliant ATP you may self refer to the tests, I believe. The tests focus on minutiae American pilots would never consider: "what is the frequency of the super heterodyne receiver in the backup module of the vortex generator when the entertainment system powers up on a second sequential reboot?"
Typical advice here and on Reddit is that financially it makes little sense for a competitive American pilot to pursue an airline job in Europe. "CFIs make more than new FOs" is a common comparison.