The documentation for getting a blog built in 15 minutes doesn't explain why you put your code in each place or how it's put together, because that's not the point. Rails is about making default decisions for commonly bikeshedded questions like "how do we name our database tables" so you don't waste mental bandwidth on them when you're trying to build an application. But explanations for how and why all that happens is very readily available.
The documentation for getting a blog built in 15 minutes doesn't explain why you put your code in each place or how it's put together, because that's not the point. Rails is about making default decisions for commonly bikeshedded questions like "how do we name our database tables" so you don't waste mental bandwidth on them when you're trying to build an application. But explanations for how and why all that happens is very readily available.