It’s a type of visa with benefits afforded a more temporary form.

A green card like a visa can be revoked. Citizenship gets a bit more interesting with the current administration.

A green card is literally not a visa in US Law.

In other contexts, it literally is.

Correct and it's a typical process for many countries. You get a visa so you can stay past the tourist time (often 90 days and with an intent to apply for residency), and then while in country you apply for residency (the green card). One of the issues in the US is the process can take so long you end up overstaying your visa. Logically you should be able to extend the visa or the gov. shouldn't care since it's their fault it's taking so long. But, it's better for one side to simply declare these people illegal.