slim4-skeleton VS no-framework-tutorial

You can have the container call in a config folder with a file called dependencies or how ever you want. A few existing examples of this is here - Slim calls this app/dependencies in this skeleton project and Odan's Skeleton has this as config/container in their project.

Yes, as an example, the First Application's business logic (not that it's much) doesn't tie itself to the framework, as one example. Odan's Slim Skeleton, ADR Example (yes, I know you are the author), and the Aura Payload example class can be seen as other examples.

Even just the tests in your first link are very useful to see, such as: https://github.com/odan/slim4-skeleton/blob/master/tests/TestCase/Action/Customer/CustomerCreatorActionTest.php

Another option, to me and speaking to the OP as an alternative, it is simpler to add some Data Mappers/Repositories to abstract away the Database code. Good examples is the sample ADR application, Payload library example, Slim 3 First Application, or this Slim 4 Skeleton application

You could also use a Slim Skeleton like Odan's which incorporates much more features to help - docs.

Try Slim again. The setup may be too much, but the concept is the same. Look at the first application or any of the skeleton apps to help (https://github.com/odan/slim4-skeleton for one).

I myself learned by following the great "no-framework-tutorial": https://github.com/PatrickLouys/no-framework-tutorial Disclaimer: The last one is about 7 years old, so some things have changed, but all in all the information there is still valid.

No framework tutorial

Here's an all-in-one github repo that I found to be a nice build for the fundamentals: PatrickLouys/no-framework-tutorial

Use libraries or a micro framework if you are hesitant on using a full framework. Nothing wrong with Slim and some other libraries. Or bundle some libraries together to make your own slim (pun intended) framework - like this or this.

https://github.com/PatrickLouys/no-framework-tutorial also got turned into a book.

This tutorial (and subsequently the book) helped me a lot when learning OOPHP.

Do you already know how backends work? Maybe this can be a good read. Otherwise just look up the basic syntax and some quirks and you will be fine.

Search for "PHP without a framework" to find articles such as https://github.com/PatrickLouys/no-framework-tutorial that will show you how to "fill in the blanks" with libraries or simple code, or use a "microframework" such as Slim.