Short answer
HubSpot's KB: most data sync apps map associations by default. For some apps, such as Airtable and Kintone, and for custom objects, all syncs must be activated before associations are recognized.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Set up the company sync first, then map associations in the contact sync.
3. Related
Field mappings and custom object sync.