In practice it's as simple as doing a composer update for the most part - assuming you're configured "sensibly" - the RoundEarth setup makes (or at least made!) some assumptions re: versioning that are not helpful for upgrading. So here goes:
You should declare both civicrm-core and civicrm-drupal-8 in your composer json file.
You should have the same ...
My experience with anything composer-ish seems to be:
Struggle with it for a while until the site becomes messed up beyond repair.
Restore from backup and struggle some more until it's messed up again.
Create a brand new install and then copy over the sites/default folder and .htaccess and such.
Replace the database with my backup.
Run the upgrades.
Somebody who is searching for this answer can use the following civcrm extensions.
I have not tested "Activity Calendars" though.