5.1 Use Composer

If there is a new version of a module and you want to update to it.

composer update drupal/admin_toolbar

That fetches the module code. To run the updates run:

drupal update:execute admin_toolbar

And commit your changes (export your site config first...)

git add | commit | push | merge

5.2 Sync your local repo with remote

If other developers on your team have updated modules for the repo you are working on, you can fetch those changes using:

git fetch | merge

and

composer install

and

drupal update:execute admin_toolbar

or, because you don't know what has been updated...

drupal update:execute all