After discussion on who could be a captain, I would like to clarify, is having ATPL a requirement for PIC on an aircraft that requires two pilots to fly?
Negative. A person holding a private pilot certificate (at least in the USA) is allowed to add a type rating granting both PIC and SIC privileges in an aircraft type certified for a two person flight crew provided the flights are not carrying passengers or property for compensation or hire. See 14 CFR 61.117.
An ATPL would be required if said type rated pilot was conducting Part 121 or Part 135 operations in such an aircraft.
An example of not needing an ATP is one pilot flying under the hood doing instrument training, and the 2nd pilot watching for traffic and monitoring approaches perhaps. Both can be Private Pilot, Airplane Single Engine Land, about as minimally rated as it goes (not counting the light aircraft/sport aircraft stuff, as I don't know the airspace limitations on those).