So far I do not have all the answers, but the few that I know first hand is yes you operate on their license. during screening, other than medical, you do a sim eval in the -400.
a written test for ATPL.
this process is about 10 days or two 4 or 5 day sessions.Depends on your
contractor.
then eventually a checkride with the CAAC for your ATP rating and license.
this is there procedure for qualifying you for their ATP, then you operate on their certificate license.
if you are -400 rated they train you at Boeing on 777.