I'm running three separate "groups" of players through a home-brew/Greyhawk campaign via pbem mostly. They're all roughly in the same period of time (same month, year), but in different parts of Greyhawk. So, they might hear of one another, or their exploits, or ideally, they'll all end up joining up into one party.
So I answered multiple campaigns in the same world AND home brew, even though I'm using the Greyhawk Map and gold box background and Ashes, I'm using Beyond the Wall character classes, skills, and magic system, with OSE underpinning it all.
I also answered we usually start a new campaign each time - when I was a player, I never remember our forever DM linking campaigns in any way. Once we were "done" or the game petered out, we started a new game. So if we were playing FR, the next might have been Planescape, then Al Qadim, then the next one. I ran a Moonshaes campaign in 5e, then Greyhawk. I might use the same "world" but prefer to move to different areas, when I DM.