Why "DET er"? I thought "det" means "it". Can this sentence mean "it is tea in the cup"? Is "det er" always "there are"?

Could you use "det finnes" here?

Normally I don't say 'det finnes ....' in this context. Det finnes te i koppen is very strange to me.

This may be a silly question, but would "I koppen er det te" work here? There is a similar line about wine in a glass in this section which (if I remember correctly) reads, "I glasset er det vin".

Is there any difference between 'det er' and 'den er'?

What is the difference between 'der' and 'det' ?

Are i and på interchangeable?