How to store mnemonics in hd wallets?

Question

Data storage

I am developing a crypto wallet, I learned how to create a mnemonic phrase, the question arose of safe storage of this phrase, where is it better to store it, given that the application is written on android, there is an idea to make a separate service for creating and transmitting these keys, but there is a fear that the request that goes between the user and the service may intercept, tell me how to store and how to transmit.