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.

It depends a lot on the series, for exanple Modern Warfare usually has related campaigns, for both its modern and original trilogies. Bkack ops is way more unrelated

They're related. Plus Warzone season trailers also count as lore.