What exactly is a Nintendo Account?

I wanted to ask what exactly is a Nintendo Account.

I bought a Nintendo Switch Lite about a month ago. But I haven't created a Nintendo Account there just a game account.

I wanted to know what the advantages of having a Nintendo Account, whether it cost what, and how exactly you created one.

Since I'm generally very unsure about technology, if I do something wrong, I wanted to ask.

With the Nintendo Account you can download games from the e-shop.

And get a friend code ''

And then you can play online.

For online gaming you need such a card for 20 euro a year.