https://kotaku.com/cod-black-ops-6-blops-zombies-solo-features-leak-rounds-1851616861
The upcoming Call of Duty from Treyarch will also let you join zombie games already in progress, too
Made by Treyarch and also:
One of the biggest new features added to Treyarch’s round-based Zombies mode is the ability to take a break during solo runs and return to your game later. This will let players who might be getting tired or who need to eat something stop and save their progress, and come back to continue their run when they’re ready.
Another cool feature added to this year’s CoD Zombies mode is the ability for friends to jump into your co-op match while it’s in progress. Perhaps you’re playing with a friend but they have to leave, or their internet connection drops. Well, now you can invite another friend to hop in and help out without having to restart the run. (This is only available in co-op matches, so a solo player can’t hit round 100, take a break, and call in some friends to help later.)