"Are you comfortable?"
Translation:Har du det bekvämt?
There has to be something in the sentence which is comfortable (a sofa, a situation, who knows), so you just insert a dummy pronoun ”it” which is the abstract thing which is comfortable. It’s called dummy pronoun or formal subject, much like it in ’it is raining’.
There are some adjectives that use this have-construction, and then you have to have the det. Another example is jag har det bra lit. ’have it good’ which means something like ”I am happy/comfortable” or like ”the situation is good for me”. It can also be used when bidding farewell: Ha det bra! (Have a nice day/trip etc.)
I am not 100% on this but comfort is not something you are, it is something you have in Swedish. In French one is not hungry, one has hunger. I believe it is much the same in that respect.
Can I use "Är du bekväm" and "Har du det bekvämt" interchangeably or is one better to use than the other for certain contexts?
You can use them interchangeably but the latter sounds a lot more natural to me. I don't know if other native spears might disagree, though.