The formal rules of English grammar would require one to use "whom" with did or do. In those sentences "you" is the subject, and seeing is the action being done (verb) to "whom" (the object).

You could ask a prosaic AI (such as any version of the ChatGPT LLM) to diagram sentences to check other examples.

Formally, you should use "whom."

But this will sound awkwardly pedantic to native English speakers (anglophones). Mostly, we only use "whom" where its preceded by a preposition such as "of" or "in."

Of whom were you speaking? In whom would you place your trust?

A further complication is that the primary verb here is "see." But we're subordinating that with the "auxiliary" verb "did."

You did something. (What?: you saw.) To whom did you do that?

Who was it that you saw? Whom did you see?

But, for more than 95% of Americans in casual conversation, whom will sound weird in that properly worded second example.