A wide variety of Adoptionist Christians are attested in the early Christian times from various sources. Among them were both the Jewish-Christian groups such as the Ebionites, and some Gentile Christians, such as the followers of the "heretical teacher" Theodotus of Byzantium who was active in Rome at the end of the second century. So the Adoptionists' beliefs were clearly far from uniform.
In general, two types of Adoptionism are found in our earliest sources:
the Resurrection-Adoptionism, and the Baptism-Adoptionism.
The Resurrection-Adoptionist Christians believed that Jesus became the Son of God only at the moment of his Resurrection, Theodotus claimed that Jesus was born of the Virgin Mary and the Holy Spirit as a non-divine man, and though later "adopted" by God upon baptism, was not himself God until after his resurrection.
The Baptism-Adoptionists saw the moment of the Baptism of Jesus as a big turning point. Both these types of Adoptionism are well attested in the NT, and this should indicate that the roots of Adoptionism may indeed go back to the most primitive layers of the Christian tradition.
Of course the present orthodox dogma, formulated in 325 AD at Nicea, insists that Jesus was both man and God at the same time.
The Adoptionist creeds could belong to the "Historical Jesus" side.
And the question of how God became Jesus belongs to the "Mythical Jesus" side.