Short answer
HubSpot's KB: HubSpot forms and non-HubSpot forms set contacts as marketing by default; chatflows set them as non-marketing. You can change the default per form or chatflow. Some tools have a fixed default that can't be changed.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Review form settings before a campaign to avoid surprise marketing contacts.
3. Related
Connected apps and non-marketing syncs.