There are three places that can affect automatic Zoom emails for sessions created through HeySummit.
HeySummit's Zoom host setting: In HeySummit, go to Event Setup > Webinar Hosts, open your connected Zoom host, and use Ask Zoom to send registration confirmation emails?. By default, HeySummit keeps this off, which means HeySummit asks Zoom not to send registrant confirmation emails when attendees are registered with Zoom.
Zoom account-level webinar email settings: This is usually the best place to disable Zoom's own automatic webinar emails globally, because it applies to webinars created in the account unless a webinar overrides those defaults.
Individual Zoom webinar settings: Use this only when a specific webinar needs different email behavior, because you may need to adjust it again after HeySummit creates or updates the webinar.
For the Zoom account-level settings, open Zoom and go to Account Management > Account Settings > Webinar > Emails. From there, turn off the automatic emails you do not want Zoom to send.
If you prefer to change email behavior for one webinar only, open that webinar in Zoom and adjust its email settings directly. This can be useful for exceptions, but it is more manual because each newly created webinar may need to be checked.
For most HeySummit events, keep Zoom's automatic registrant emails off and use HeySummit's attendee emails instead, so attendees receive one clear set of event communications.