Yeah the owners and service manuals are two very very different things. You won't be performing much maintenance with the owners manual besides the absolute basics, like checking your oil.

The cheap OBD2 readers lack the vast majority of data and tools you would want to use for service as well.

There are aftermarket options but most manufacturers want you to subscribe to their service tools - often charged per monthly plus an additional a fee per VIN it touches. These cost in the hundreds to thousands. It can be pretty frustrating as a shadetree mechanic.

Sadly service manuals aren’t commonly sold anymore and you need a subscription to a website to access factory service manuals.

It’s a shame, I use to buy the service manual when I bought the car.

Reading codes is only part of it. For the last quarter century or so (some manufacturers more than others) if you replace certain components (radio, any computer module) you need to update the ECU to recognize the new component. This requires proprietary software and in many cases proprietary hardware as well. These "features" are marketed as an "anti-theft" mechanism. This renders some repairs basically impossible for the end user to perform.

The fact that they've been allowed to build systems like that is appalling.