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