Please note that this app ("Onion Browser") is not affiliated with the official Tor project [1], and their website never clarifies that anywhere. Intentionally very misleading, and leaves a bad taste in my mouth.
The code does appear to be open source, but on iOS devices there is NO WAY to ensure the code that's on the GitHub page [2] is what's running on your device, and sideloading isn't a realistic option either.
[2]: https://github.com/onionbrowser/onionbrowser
CORRECTION: As child comments point out, it appears that this app is loosely affiliated with the Tor project, and is recommended by them.