What is the historical basis for the numerous Buddhas?

As far as I understand it, there was one historic individual, who is appropriately referred to as the "historic Buddha" in scholarship, who has, if you believe in it, achieved Enlightenment. However, different branches/schools of Buddhism also hold a bewildering array of other Buddhas and Bodhisattvas, prominent named examples of which, off the top of my head, are Avalokiteshvara, Amoghasiddhi, Tara, Manjusri, and Maitreya. Numerous others are believed in.

How did Buddhists get the idea that these enlightened individuals exist? The prominence of "apotheosis" in Buddhism kind of makes it difficult for me to understand where the human prophets end, and deities begin. Do they correspond to any historic personages, like the historic Buddha does, and prophets and religious figures of other religions do? Are they just purely imaginary (for lack of a better term), like the gods of Hinduism, Norse and Greek mythology, etc.?

If they do not correspond to accounts of historic personages, what epistemological account does Buddhism give of how these entities entered into their belief system? Did they appear to Buddhist monks in meditation?

Does Buddhism have a system akin to canonization in Catholicism, where some body of religious experts admits certain entities into the pantheon of Buddhas and Bodhisattvas?