git request-pull
Generates a pretty email requesting someone to pull commits from your online repository. It's really meant for Linus to pull a whole bunch of already-reviewed changes from a maintainer's integration branch.
The rough equivalent to GitHub's "pull request" is the "patch series", produced by:
git format-patch
Which lets you provide a "cover letter" (PR description), and formats each commit as a diff that can be quoted inline in an email reply for code review.
You are right, PRs are not in git. format-patch and request-pull are originals designed for mailings lists. Github just put UI on top.
Interesting that DAG model means any branch from anywhere can be merged... the forge is just coordination.
Explored here if curious - https://vectree.io/c/git-graph-theory-logic