All new records or changes made by HubSpot that appear in Salesforce use the credentials entered in your HubSpot-Salesforce settings. Any change made via the integration between HubSpot and Salesforce is as if this user is logged into Salesforce making that change. This is true for all objects supported by the HubSpot-Salesforce integration: Leads, Contacts, Accounts, Opportunities, and Tasks.
All intereactions with Salesforce via Hubspot will look as if made by the Integration User -- the user whose credentials are used to login to Salesforce from the HubSpot-Salesforce settings.
A task may be 'assigned' to another Salesforce user once it syncs over to Salesforce from HubSpot, but the 'CreatedById' or 'LastModifiedById' is always the integration user. The integration user is the one whose credentials are sending the data from HubSpot to Salesforce, so it's their ID that shows up there.
'Created by' is also a read-only object in Salesforce so it cannot be modified.