Ranzie's Overgrown Cities Add-on 1.21+

If you want to explore something new and dangerous in Minecraft, you should add abandoned cities to your world. The mod Ranzie's Overgrown Cities creates large ruined cities that have become overgrown with trees and vines over time. Inside the buildings, you can find useful items, but danger often hides there as well. Each location looks different, making exploration more interesting and a bit more intense.

All of this will generate right in your world.