That's it. That's exactly how it works. People act like using whom instead of who is fancy, but it's really no more complicated than knowing the difference between he and him or she and her.

I just assume that if what you're referring to is "them", it's "whom".

I cannot learn otherwise, I am too stupid.

Is it the thing that's doing the verb? Then it's who. Otherwise whom.

It's always great to learn more. Next you can relearn colons.

"In modern English usage, a complete sentence precedes a colon, while a list, description, explanation, or definition follows it." https://en.m.wikipedia.org/wiki/Colon_(punctuation)

Bork, you're a Federal Agent. You represent the United States government. Never end a sentence with a preposition!

Ending sentences with prepositions is something up with which I will not put!

It's fine to end a sentence with a prepositional or phrasal verb. https://www.grammarly.com/blog/end-sentence-preposition/

You can end a sentence in a preposition, and whom isn't a preposition anyway. It's a pronoun.

"Of her, I am afraid."

Great, now I'm sounds like Yoda.