Project Zomboid

Project Zomboid is an open-world sandbox, with the main goal being “Survive.” Around since 2011, it received ten years of updates to create one of the most well-loved zombie survival sims. It’s still alive and well, because despite its last major update happening in 2021, it is buoyed by a lively modding community and an involved development team.
The game doesn’t tell you how to do anything, so there is a bit of a learning curve, but there’s a ton of documentation out there. Plus, you can play online with your buddies, creating your own post-apocalyptic idyll.