Short answer
HubSpot's KB: sales emails, tasks, meetings, notes and content views sync only after the contact syncs; earlier activity doesn't. Marketing email opens and clicks are deduplicated per type on the timeline. Pick a sync direction for associations.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Sync contacts early if you want their history in Salesforce.
3. Related
Sync rules and the connector package.