Unlike the rest of humanity, Izuku didn't unlock his quirk at the age of 4, but instead, at the age of 10. A device appeared on Izuku's wrist. The name of this device was the Omnitrix, and it allowed him to turn into any Alien imaginable. Follow the...
Omnitrix functions: Ultimatrix- Allows any Alien of Izuku's choosing to go through a simulation of 10 million years and give them a more powerful form based on the best outcome of the simulation.
Biomnitrix- Allows Izuku to only fuse two aliens, which are then saved into the Omnitrix's system allowing Izuku to use the combined Alien by its lonesome without having to fuse the two Aliens every time he wants to use the fused Alien.