"View post since last visit" mechanics

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What I want to ask is basically, does anyone know about the thread title, that is, how does WP know what to show when I click into "View post since last visit"? Like, I know that if I am in WP in one device and then access from another one it resets in both, or when I switch my IP direction it also resets even if I am on the same device, and it eventually resets on it's own if I access WP and then am away for a while, but basically, is that information kept strictly server side, or strictly from cookies in the computer , or a relationship between both of them? Mainly I just want to know, but there are times it resets for apparently no reason and I wonder whether I could tweak some timestamp in some cookie to un-reset it or something.