What are the obligations of a US corporation when a vote for unionization is successful? Do all employees get opted in? Is it always required to be hiring exclu

What are the obligations of a US corporation when a vote for unionization is successful? Do all employees get opted in? Is it always required to be hiring exclusive to the union? What makes unions different (needing specialized legislation) from corporations - why can’t employees wanting to unionize just go form their own corporation and try to sell their services on their own terms that way.