Yes, most things on Replit are open-source. Just click "show files" on the top-left corner. Also it's powered by Nix, and we wrote a ton about it here: https://blog.replit.com/
It's not clear to me what code I'm running.
1. What is the secure (?) frontend? Is this xterm?
2. What is the outer GUI operating system? How can I interact with it?
Even for hobbies, I don't like building on something I don't understand.