Resolution Center
Resolution Center is the message thread inside App Store Connect where App Review sends rejection notices and you reply to them.
It is attached to a specific version, and it is the only channel where the reviewer who looked at your app will read what you write. Support tickets, developer forums and the general contact form go elsewhere and to other people. If the rejection arrived by email, the email is a notification; the thread is the conversation.
A rejection there has three parts worth reading separately. The guideline number, which tells you which rule they applied. The body text, which is what they actually saw. And often an attachment, usually a screenshot or a screen recording of the exact moment. The attachment is the most useful thing in the message and the most commonly ignored.
A reply that works is short and specific. Say what you changed, name the screen or field, and answer the question they asked rather than the one you would prefer. Attach a screenshot or a short recording showing the fixed behaviour. Include working demo credentials again even if you supplied them before, because reviewers change between rounds.
What it will not do is escalate. Reviewers can hold a position across several rounds, and there is no button that gets you a different one. Disagreeing on interpretation is what the App Review Board exists for.
See also: metadata rejection, App Review Board, expedited review.