I read it wrong the first time. I thought it was like "Go home Florida, it full" like "Go home Florida, you're drunk"

Kinda looks like color banding

"So I'm a Spider, So What?" but an entire city rather than an entire class?

Lots of options here, like RPG-style where everyone is basically the same age as at death, but suddenly appearing and functional crippling the economy and probably starting multiple civil wars. Coming all in as newborns, so likely a larger number being monsters, though many very quickly dying off.

Also makes me think of the scale of the world itself, or if there's a sort of buffer where folks appear over time. Now that I think of it, this might be a bit more like "Log Horizon" but other than knowledge, everyone is effectively useless.

As someone who has tried to make a similar sandwich, there's a good reason why it ended up becoming an open-face sandwich; it just ends up squeezing+falling out everywhere when you try and take a bite.