If I have multiple GrowSurf campaigns connected to the same integrations, how does data syncing work for my contacts?

The contact data would be overwritten. For example, if participants exist in Campaign A and were subsequently added to a HubSpot for Campaign A, and then later is added to Campaign B along with a HubSpot for campaign B, the participants' information from Campaign B would override that of Campaign A (this would apply to all integrations).