Where I can find a tutorial which explains step by step creation of an app using React?

Maybe it's a silly and redundant question, but I hope to find someone who can help me. I'm searching for a tutorial where i don't need just copy a piece of code and see the result. I mean a tutorial or something like that where I can find the explanation of every snippet of code(i.e. if I use array .map, why I use that? For what reason? Why in this case? And so on...), the reason why it's better choose one formula or the workflow I need to follow when I approach with React.

Hoping not to be boring.

Cheers!