I do not understand how a person can get green card by working in the USA. I though a person can only get visa but no green card. Am I wrong?

You got the terms mixed up. This is my understanding of the terms:

- The visa is something you need to enter the U.S. A visa is only issued abroad at the embassy or consulate. You will need a visa to enter the U.S. If you change status in the U.S. and never leave the country for any purpose, you will not need a visa.

- So, say that a person enters the U.S. as a student on the F-1 visa. The changes status to H-1B and never leaves the U.S. during the working period. The company then sponsors the student/employee for a green card. Once the person gets the green card, he or she will never need a visa anymore to enter the U.S. (some exceptions apply, but in general never). If the person later applies for US citizenship, then a visa will never be needed as a passport is the only thing you would need to travel abroad.

I hope this helps and answers your questions.