if you are familiar with object vs subject in grammar you already know the rule, who is used when it’s the subject, whom when the object:

Who is that?

That’s who ate my ice cream.

Whom did you give ice cream to?

The ice cream went to the one whom I saw first.

This rule is the same as knowing when to use she or he vs when to use her or him, it’s no different.

However, most people don’t use whom correctly and it can just be avoided entirely, most people will just use who as the object anyway and it will sound more natural to them:

Who did you give ice cream to?

The ice cream went to the one who I saw first.

Using whom in these cases can make you sound formal or fancy, and draws attention.