"Yes, you are welcome."
This is the literal meaning, not the real use. You return the politeness, not the thanks. Returning the courtesy of the thanks is a way to say "It's nothing, small thing, it doesn't worth to say thank you".
Sama means literally "same", so it's a close meaning.
When you say "you are welcome" in English, it makes no sense neither to consider the literal meaning, it's just an expression.
If you analyze the literal meaning of "terima kasih" (=thank you), "terima" means to accept/to receive and "kasih" is the fact to like something. So, literally Indonesian say "I receive the pleasure" when they say "thank you".
And saying "kembali" makes sense, as you return this please, like saying:
-It was a please.
-No, the pleasure was mine.
I don't think the English expression is really that meaningless, though. I at least think of it as "you are welcome to thank me if that's what you want" XD ... Or maybe as a metonymic for "your gratitude is welcome"
Anyway, you have other alternatives like "don't mention it / my pleasure / anytime," etc. I think these are closer to what you find in many other languages.
I've never heard anyone say, "Iya sama-sama". Only "sama-sama".
Also - why is "iya" accepted on some as "Ya" and not on others..?
I write "iya, terima kasih" on another and it's accepted as alternative solution - not here.
Iya is correct. It also helps differentiate (phonetically) from "Ja/Yes" in German and "Я/I" in Russian, for any polyglots - "ya" is a teenage girl from the USA who just started their freshman year trying to sound cool when they are, "ya, like, totally?".
It's not supposed to be a full expression. It's Yes. You are welcome.
And the "yes" is responding to a question.
- Are you happy to see me again?
- Yes! Welcome!
They didn't say it was supposed to be an expression here. It's the part of a dialogue.
The proper word is "iya" (yes), but it's very common to find "ya" as an abbreviation, it's a lot less formal.
If you want to compare, "iya" would be a little like "yes", and "ya" like "yeah". Like the distinction between "thank you" and "thanks".
They are very consistent, they just want you to learn the 2 way to say "yes".
You will complain, outside of this couse, if you had only "iya", that people use a lot "ya", and you didn't learn it. Or the reverse.