[2021] Help me to understand gamestaging...

I think jacques has it right. You’ll get proportionally more of the spitting cops, demolishers, and irradiated zombies. There will be more zombies in total, but you’ve likely already hit the limit and are getting zombies right up until morning already. If you’re shooting them, you’ll find increasing pressure to come up with more ammo: brass in particular.

When you have ‘beaten’ the game is subjective. It’s a sandbox, and the story that might provide an end cap to your adventure isn’t implemented yet. You can make challenges for yourself, and/or turn up the settings, and/or try mods. Have fun!