[git] How to compare a local git branch with its remote branch?

First type

git branch -a

to get the list of available branches. On the output you may see something like

* master
  remotes/main/master
  remotes/origin/HEAD -> origin/master
  remotes/origin/master
  remotes/origin/mt
  remotes/upstream/master
  remotes/upstream/mt

Then show the diff

git diff --stat --color remotes/main/master..origin/master
git diff remotes/main/master..origin/master