The Greater Delphia Universe started as a simple mind exercise to give the creator's characters a place to exist in. Originally a plain void where they gathered, it turned into a lodging house near a set of woods, then into more of a city, then a megalopolis. And now 3 years later, and with assistance from the creator's partner, it's developed into an entire solar system, and even has a spinoff of an Undertale AU!
After three years, many trials and tribulations, a TikTok page, and more, the Greater Delphia Universe now has an online repository for all of its history, including the previous eras; the beta tests, if you choose. Feel free to explore, there's a whole world to uncover!
Good question! There are quite a few places to start your perusing here.
If you want to learn more about the characters that started this whole thought experiment, visit Zed's Characters. They'll be sorted by recency.
If you want to learn more about the worlds they live in, visit Terra Hominus - Earth and Terra Daemonium.
If you want to learn more about the creators, visit the About page.