Some of them are related. Some of them aren't.

Black Ops and Modern Warfare are generally two separate series- the Modern Warfare games are all related.

Black Ops is a lot more complicated. Black Ops 2 is a direct sequel to Black Ops 1. Technically Black Ops 1 is sort of a sequel to World At War, as well. They share a major character, but it's kind of a minor thing and you won't be missing a ton.

Black Ops 3 has basically nothing to do with the rest- it takes place in 2065 and basically the only thing that links it to the previous games is a throwaway line related to a previous villain and some text logs.

Black Ops 4 didn't have a singleplayer campaign.

Black Ops 5 is Black Ops: Cold War, it is related to 1 & 2 but it's less of a direct continuation and more just the same characters are involved iirc.

Black Ops 6 follows up Cold War, but again is just the same characters.

Personally, I'd suggest doing World At War, Black Ops 1, Black Ops 2, Cold War, and then Black Ops 6 for the 'full experience.' If you wanna circle back around and do Black Ops 3, you can do that pretty much whenever because as I said it's unrelated. You can drop World At War if it doesn't interest you without any real issues. As I said, it just sets up a single character. Dropping any of the others you might actually be confused on plot and characters at points, idk.

Modern Warfare is a lot simpler. Just do them in order.