Learn Swedish: Att grannen på bottenvåningen aldrig stänger dörren! - I can't believe that the neighbor on the ground floor never closes the door!

This is an independent exclamatory att-clause. Swedish lets you drop the main clause (like "It is unbelievable...") and just use the subordinate clause to express strong feelings like frustration, shock, or indignation. The tone of your voice and the exclamation mark do the rest of the work.

Why is the word order aldrig stänger instead of stänger aldrig?

Because this is grammatically a subordinate clause, it strictly follows the BIFF rule. This rule dictates that sentence adverbs like aldrig (never) or inte (not) must be placed before the finite (conjugated) verb. If it were a normal main clause, it would be Grannen stänger aldrig dörren.

What does the word bottenvåningen literally mean?

It is a compound of botten (bottom) and våningen (the floor or story). While English says "ground floor", Swedish literally says "bottom floor". Note that you cannot use grund (ground/foundation) here; grundvåning is not a standard Swedish word.

Could I say sin dörr instead of just dörren at the end of the sentence?

You could, but it changes the context. Sin dörr would specifically mean the neighbor's own apartment door. Using just the definite form dörren (the door) in this complaint usually implies a shared door, such as the main entrance to the apartment building, which everyone expects to be kept closed.