I believe their desktop and Android wallets are open source, but iOS isn't. Check out their github for more details.

I use Nunchuk as my primary on-chain phone wallet along with a Tap Signer. I like their UI of separating keys and wallets and how they make multi-sig setup very easy. I also like their NFC support of Tap Signer, Sats Card, Cold Card. I don't use their shared custody service though, and use the wallet solely for self custody.