There are many possibilities.
You got it.
Student or Learner
My nine-year-old god daughter's English book teaches them the following dialogue in a restaurant:
Tom Hi! Can I have a pasta, salad and cheese, please?
Waiter Certainly. And to drink?
Tom Can I have water, please?
Waiter Pasta, salad, cheese and water. That's fine.
There are a couple more similar dialogues but each one is ened with a "That's fine." said by the waiter.
Is that a natural thing to say in this context? Maybe I haven't been to a restaurant in an English-speaking country that many times but I don't remember ever hearing that in such context. I would rather imagine the customer saying it to confirm if the waiter got the order right.
Could anyone please tell me if it's really how it is said?
It is said by a waiter for several reasons, to make sure that they have your order correct and confirm that all the items you have ordered are available.
I also find the textbook dialogue slightly artificial. Most are, however. I would not use "that's fine" in that situation, but I suppose it's not impossible.
They don't literally ask you if they have the details of your order correct so the interrogative isn't necessary. They simply repeat the order back and expect you to say something if they've got it wrong.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
I think that using "that's fine" is a little strange. The customer didn't ask if it was ok to order any one of those things. But what's also stange is how the customer orders "a pasta" when it would sound better to just say "pasta".
And the part about the cheese is also odd.
The cheese plate? Okay
A hamburger with cheese? Okay.
But the way it's presented here? Odd.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.