It’s built with React Native (an open source project developed by facebook) and Breez Liquid SDK (which uses Boltz submarine swaps). You can read more about it here: https://blog.onionmill.com/p/introducing-stashpay-a-bitcoin-wallet
I want to see the source code
why not build the app with Android Java/Kotlin for Android and Objective C for iOS? No reason for a app like that to be over 170mb if all you doing is using Boltz api for swap.