Your question seems to imply that the Buddha actually taught Mahayana sutras. I would recommend reading previous discussions on this forum.
For example: Regarding the belief of academics that the Buddha taught Mahayana sutras etc
On Wikipedia you can find some origin theories about Mahayana: Mahayana - Wikipedia
And for a more academic perspective: (PDF) Early Indian Mahayana Buddhism I: Recent scholarship | David Drewes - Academia.edu