Learn Russian: Их улицу затопило, так что они переехали.

This is an impersonal construction often used for natural phenomena or disasters. The unspoken subject is the natural force itself (like an invisible 'it'), which takes the neuter verb затопило. The street is the direct object receiving the action, so улица changes to the accusative улицу. Literally, it means 'It flooded their street.'

Could I use поэтому instead of так что in this sentence?

Yes, they have very similar meanings, but they function differently in the grammar. Так что (so) is a conjunction that introduces a subordinate clause of consequence, linking it directly to the first event. Поэтому (therefore) is an adverb and typically starts a new independent clause or a completely new sentence. Так что is very common in everyday speech to show a direct, factual result.

Where exactly does the comma go when using так что for consequence?

When так что means 'so' or 'with the result that', it acts as a single conjunction block. The comma always goes right before так, separating the main clause from the subordinate clause. You should not put a comma between так and что in this context.

Why is the verb переехали used here for 'moved'?