Each of these Buddhas was in turn assisted by certain bodhisattvas. Amitabha was assisted by two bodhisattvas, Avalokitesvara and Mahasthamaprapta.
These Buddhas of the eight heavenly universes were different from the historical Buddha, Gautama Buddha, who lived and died in India (to which Theravada Buddhism traces its origins).
Unlike the earthly Buddha, these cosmic Buddhas lives forever. They are each effectively like a god controlling a heaven, with the bodhisattvas helping them to rescue humans from suffering.