Reviewers and assigns are a tool for organizing someone who works on what a request and a problem require. They do not provide any additional permissions.
Reviewers are the people you specifically request to view your pull request. From the documents ...
Request authors and request owners and copywriters can request an overview of the transfer request from any user who has write access to the repository. The requested reviewer will receive a notification that you have asked them to review the transfer request.
While "copyright holders specify who works on specific problems and pulls requests", also from the documents .
The specific difference depends on the project, but the main difference is what reviewers are expected to review, and assignees are responsible for processing the transaction (which probably also includes a review).
For example, if I submit a transfer request that has changed the type system, I could assign it to the type system responsible to take care; it can be a specific person or project. Or I can assign it to those who say that they will take care of this in the project chat. The Assignee says that "this person is dealing with a problem."
But if this pull request also contains Windows code, I can ask for feedback from someone from a project who knows Windows well.
Another use may be to always have two people who are looking for a pull request, one reviewer, one assignee. In this case, the project manager will control that each transfer request has both an assignee and a reviewer.