Not a native speaker, but I think you ask for the subject with ‘who’ (“Who did this?”) and ask for the object with ‘whom’ (“To whom was it done?”).

Anyways, owl set blud up man

It can get incredibly complicated so I don’t understand all the nuances myself, but I’ll explain it as best I can: Prepositions are words that denote where things are in space and/or over time. Things like “of” or “from” are common prepositions.

Prepositions are often followed by an Object. So if I said, “A letter arrived from Jeff,” Jeff would be the Object of the Preposition “from.”

If you want to ask the Identity of the person who sent the letter, the formal, “proper” English sentence would be: “From whom did we receive the letter?”

You could also ask “Who’d we get the letter from?” BUT, old-fashion English grammar rules say that you are not supposed to end a sentence with a preposition. In modern times however, such rules aren’t really observed. It seems our bartender is a old fashioned sort.