https://www.grammarly.com/blog/who-vs-whom-its-not-as-complicated-as-you-might-think/
^Good link. “Whom” is the objective form of the pronoun, like “him”, “me”, etc. “Who” is the nominative/subject form.
You can think of it in terms of replacing “whom” with “him”, but know that in relative and interrogative clauses, the word order is flipped, so you will have to flip it first in order for the replacement to make any sense.
“He’s the one whom I saw…”
Flip: I saw whom. I saw him. “Whom” is correct.