adapter config in database.yml file and adapter pattern

I see Rails has a mechanism that allows us to configure the database we will use inside the database.yml file, then rails will interact with the database that we have selected in the configuration (using a design pattern which I think is the adapter pattern).

The mechanism that allows to put the configuration in a file, and deal with it with the adapter pattern might be useful in some projects. So I want to understand how it works. So I downloaded the source code of Rails to learn, but deep inside is a lot of source code and I don't know where to start.

So I hope someone with experience in this matter can give me some suggestions, thanks a lot.