Learn Swedish: Scenografin var faktiskt briljant, vilket imponerade på mig. - The set design was actually brilliant, which impressed me.

In this sentence, vilket refers back to the entire preceding clause (Scenografin var faktiskt briljant), not just a single noun. When which refers to a whole situation, action, or statement, Swedish uses vilket. If it referred to a specific noun right next to it, you would use som.

Could I use något som instead of vilket in this sentence?

Yes, absolutely! You could say något som imponerade på mig, which literally means something that impressed me. It is very common in everyday spoken Swedish and works exactly the same way as vilket here. Vilket is also perfectly fine but can sound just a little bit more formal or written.

Why do we need the preposition på after imponerade?

In Swedish, the verb imponera (to impress) requires the preposition på when you mention the person who is being impressed. You must always say imponera på någon (literally impress on someone) rather than just directly attaching the object like in English.

How exactly is faktiskt used here?

Faktiskt means actually or in fact. Just like in English, it is used here to express a slight, pleasant surprise or to emphasize that the statement is true, perhaps because the speaker did not expect the set design to be quite so good.