Most immigration is based on hiring people into jobs for which the skills exist locally but which demand higher wages than immigrants. This is called a labor shortage by employers who prefer not to think of it as a salary shortage.

This is based on my experience in working with good immigrant coders who got the job over about equally capable locals, but more cheaply. The immigrants (mostly Indian and Chinese) tended to have weaker English communication skills but stronger work ethics.

Many job reqs are scams intended to satisfy the "doesn't exist locally requirement" (superficially) so that exploitable foreigners are importable.

Nobody has 20 years of React experience, of course, but that doesn't stop people from listing this as a requirement - or a pre-selected group of people on the other end from asserting this experience.

This is just false. There are lot of reasons people hire immigrants, very few are truly incredible, singular talents. I lived with a bunch of Indian immigrants when in SF and it was universally guys not as skilled as me hired because there was a pipeline for Brahmin to the US.