Learn Swedish: Det arrangeras mycket till banketten. - A lot is being arranged for the banquet.

This is the presentational det. In Swedish, when we introduce new indefinite information (like 'a lot' happening), we often use det as a dummy subject to start the sentence. You could technically say Mycket arrangeras, but starting with Det sounds much more natural and shifts the real subject (mycket) later in the sentence.

How does the s-passive verb arrangeras work with det here?

Arrangeras is the passive form of arrangera (to arrange), created by adding an -s. Combined with the presentational det, it functions similarly to 'there is [action] happening'. This is a very common Swedish structure to describe an ongoing action when the person doing the arranging isn't important.

What is the actual subject that is being arranged in this sentence?

The logical subject is mycket (a lot). Even though Det sits in the first position to satisfy the V2 (verb-second) rule, mycket is the 'real' subject that the passive verb arrangeras is acting upon.

Why use till instead of för to say 'for the banquet'?