Short answer
HubSpot's KB: with a time frame, customer replies inside it reopen the original ticket and later replies create a new ticket linked to it. With Never reopen, replies always create a new linked ticket. A reopened ticket moves to the pipeline's first open status and Ticket reopen date updates.
1. Statuses
Ticket statuses in my demo portal.

2. "Thanks!" replies
A short time frame stops thank-you replies from reopening old tickets.