Any tips on usage of who/whom?
Who is always the subject of a verb. Whom is always a direct object, indirect object or object of a preposition.
https://www.grammarly.com/blog/who-vs-whom-its-not-as-complicated-as-you-might-think/
https://www.grammarly.com/blog/who-vs-whom-its-not-as-complicated-as-you-might-think/