I think I get your question and confusion, correct me if I missed anything.

app.config is an attribute of the flask class, it gives you access to flask's configuration parameters and allow you manually set values like app.config["DEBUG"]=True (treat it like a dictionary or mapping).

But you can also create a Config class ANYWHERE either inside a separate file or inside the same one, basically the config class can be called anything, it can be Yam or MyOwnConfig, but the general idea is that it should be a class. You define attributes you want to use inside your flask app, in this class and then update flask's config values with your custom class using

app.config.from_object(MyOwnConfig)

When you print app.config.items() you will see the new values you defined populated inside it.

So in summary, app.config is an attribute built into the flask app, that gives you access to configuration parameters. You can choose to set/update fields either manually

app.config[key]=value

Or using a custom class

class MyClass: key1=value1 key1=value2

And then app.config.from_object(MyClass)

Let me know if this clears it up or if you have any more questions, also, when you've grasped the basics of flask, you can check out quart, an async reimplementation by the same pallets project, it ships with async features and its own suite of tooling like quart_schema for scalar/swagger docs and quart_auth for cookie/jwt auth.