Work Visa and Green Cards are not the same. A "Green Card" provides permanent residency, work visa are temporary.
For instance, "[t]he H-1B Visa enables professionals in "Specialty Occupations" to make a valuable contribution to the American economy. A maximum of 65,000 H-1B Visas are issued every year. The H-1B Visa is issued for up to three years but may be extended. This provides a maximum stay of six years. The H1-B Visa holder can apply for a Green Card if a company wants to sponsor his/her application. "
https://www.usimmigrationsupport.org/visa_h1b.html