I believe that tracking pixels are just tied to something like a URL with a UUID as part of the filename/pathing. When an email is opened, the browser within

I believe that tracking pixels are just tied to something like a URL with a UUID as part of the filename/pathing. When an email is opened, the browser within the email client is told to render and that then invokes a request against a webserver hosting the tracking pixels, creative images, etc.. and that is when the tracking is marked as open in the mailflow. The email is usually modified during preparation for sending with these tracking "pixels".