Ive seen this topic mentioned on the ethers of stack exchange before and mostly these questions go unanswered but being that ionic is basically a fancy webview I am wondering if anyone has managed to implemented injected web3 using it
If you develop some Android / iOS app, you must import Web3.JS Library and call Smart Contracts from there. Ethereum TX will help you creating valid transactions to sign with Web3.JS. If someone wants to call a payable function, it's must sign the transaction to do it, so it's must puts its private key in your software.
Some people don't want to share its private keys, so you can create Wallets internally and you manage their private keys.