From what I can tell, Pylance is proprietary and intended to "extend/extinguish" the open-source VSCode codebase with Microsoft's proprietary VSCode binaries, but Pyright is open-source.

Pyright is open source (MIT)

Pyright works well for me in vim-coc. Of course ymmv

That’s because pyright was written in typescript so it makes sense to use npm to install it. Pyright has weak ide support beyond visual studio stuff. I’m pretty fond of pyright due to how responsive the developer is on GitHub. I’ve made multiple issues before and gotten an answer same day (sometimes same hour) and some small bugs I’ve reported got fixed in the next release (which happens like twice a week).

The type system is specified but the main pep people point to is the base type system. There have been multiple extension peps adding new type features and different type checkers have different progress implementing them. Also different type checkers have different bonus features. Mypy comes with plug-in support to make it easier to extend the system for very dynamic custom situations. Pyright is able to generate rough guess type stubs for untyped codebases. Pyre has the best tensor related type support likely motivated by Facebook having pytorch. Pytype I know the least about but it likely has some unique features motivated by google usage.

If the type checkers disagree on stuff defined in pep 484 that’s a bug to report. Beyond that they can vary but feel mostly similar.