Short answer
HubSpot's KB: on the Brand Kit tab, click Add Favicon. The favicon is the small image in the browser tab and only applies to content hosted on HubSpot. With the Brands add-on, set it per brand. Browsers cache favicons, so a change can take time to appear.
1. The brand kit
My demo portal has no favicon yet.
2. External sites
Sites not hosted on HubSpot set favicons in their own platform.