If you're on OPT after graduation, you can switch to an H-1B without leaving the U.S., as long as your employer sponsors you.

you don’t have to leave the U.S. if you move from a student visa to a work visa. OPT helps with the transition if you apply on time

it’s possible to stay in the U.S. and apply for a work visa right after graduation, but you need an employer willing to sponsor you.