How to update Shopware 6 to the newest or latest version using the command line interface? (yes I know there is a documentation) but is there a concise step by step guide.
It depends on how you installed Shopware.
When you installed using Zip, you can update using the zip.
You can also adjust in the composer.json the shopware/* package versions and run composer update
If you used once composer, you have to use composer for all next operations, there is no way back to zip updates.