When cookie_update is set to true (the default value), gtag.js will update cookies on each page load. This will update the cookie expiration to be set relative to the most recent visit to the site. For example, if cookie expiration is set to one week, and a user visits using the same browser every five days, the cookie expiration will be updated on each visit and so will effectively never expire.
When set to false, cookies are not updated on each page load. This has the effect of cookie expiration being relative to the first time a user visited the site.
Example of Analytics Cookie
What happens if a user's Analytics gtag.js cookie expires? I'm assuming he will get a brand new one on the next visit.
But will he receive a different user unique ID on this next brand new cookie? Or will the user's unique ID stored on the expired cookie persist on the new cookie?