Short answer
HubSpot's KB: HubSpot provisions a standard SAN SSL certificate through Google Trust Services when you connect a domain. If your site already has SSL, use pre-provisioning during connection. Certificates renew 30 days before expiry, which needs the domain CNAME to still point to HubSpot. Other certificates need the Custom SSL add-on; you can't bring a pre-existing certificate.
1. Domains
Domain settings in my demo portal.

2. Proxies
Reverse proxy domains may need a DCV record. See reverse proxies.