So if you use Open Upgrade you will find little info and the best way to do it is to give it a try a couple of times with some fake data to get you familiarized. That is what I've been doing, even tho I haven't updated the production DB because I have other work to do and other priorities... so its been a moth and still running v14 (but I successfuly updated a copy of the DB so I know I'm on the right path). Now at least I know it is possible to UPGRADE THE DB and then run that upgraded DB into another instance of odoo that runs on v15 (the custom modules need to be upgraded manually). 

I would suggest following this guide: https://www.cybrosys.com/blog/how-to-upgrade-the-odoo-15-database-using-openupgrade
As well as this other one (official one):
https://oca.github.io/OpenUpgrade/development.html
https://oca.github.io/OpenUpgrade/migration_details.html

Those were the best info souces I could find, the official one is good if you know what you are doing, and the cybrosys is good to familiarize yourself with the hole process with an example.

Finally I agree that it looks like official odoo is not too involved in the CE and upgrading from one CE version to another CE version.

Hope anything on this post will help you @Mark Monteleone, or anyone else who comes here.