Shouldnt "what is their food" also be valid?
I think if it was 'what is their food' then it would be 'apa makanan mereka'.
Apa = what?
Bagaimana = how?
Yes, it seems to be so. But in the contextual translation, "what" is also proposed for bagaimana.
Any clarification please ?
What is their food? Their food is apple, their food is noodles, their food is chicken
How is their food? Their food is good, their food is tasty, their food is moving
Yes their food is good and tasty it is also... Wait what!
Would "Berapa makanan mereka" also be correct?
Berapa is how many/how much.
It makes no sense your question.
If you want to ask the price of their food, it should be:
"Berapa harga makanan mereka?"
Can i say 'bagaimana makananya?" How is the food. Is that correct.