I'm not sure on what you actually want to know about these but the screen capture is showing linked Content Controls, not linked fields. Content Controls are a more recent feature that behave similarly to fields but have some advantages over fields.

There are good resources on the web to learn about linked content controls. I recommend you look at this page as a starting point.
Word Content Controls. This gives some background and the links at the bottom of the page provide info on the 'linked' behaviour.

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Andrew Lockton
Chrysalis Design, Melbourne Australia