Does anyone know what software runs FediDB and if it's open source? This is the most beautifully functional web interface I've ever seen.

https://github.com/fedidb/fedidb-ce

It's created by @dansup@mastodon.social who also created Pixelfed

I agree - UI is really good.

It looks like this is the closest thing to what you are looking for.

I tried to find this before and was not able to find any up to date repo.