Comment by lanakei

An immigrant visa is basically the same thing as a green card, i.e. permanent legal residency. Once you have an immigrant visa, you can enter the US and receive your green card in the mail a few weeks later with no additional work. After five years, you can apply for citizenship. You have unlimited rights to work and live in the US.

However, ignoring family-sponsored routes, it is extremely difficult to get an immigrant visa in the first place, usually requiring years + $10k+ of fees to work your way up to it. You also need a sponsor to pay the fees (legally, you can't pay it yourself). Therefore, the vast majority of people start on a non-immigrant (temporary, restricted employment) visa and eventually ask their employer to sponsor a green card.

When people say "if you wanted to immigrate, you should just get an immigrant visa", they usually assume any other route is a hack or loophole. But it's actually the most common way to immigrate by far. You can, of course, interview for a job from abroad and ask the employer to directly sponsor an immigrant visa, but they'd have to wait years (best case) until you could actually step foot in their office. Plus they'd be forking over thousands in legal fees for an employee they haven't even seen in person. Nobody would do this, so the commonly accepted way is to bring an employee over as a temporary worker first and then apply for a green card while they're in the country.

By the way, the same checks apply for immigrant visa applications both inside and outside the US. You might think that employers are scamming the government by turning purported temporary immigrants into permanent ones, but the exact same qualifications checks, eligibility requirements, waitlists and quotas apply if you do the process inside vs. outside the US. It's entirely possible, and common, for green card applications to get denied (and the applicant's location doesn't factor in to this).