How to build a Crypto Wallet integration app?

Could you elaborate on the steps involved in building a crypto wallet integration app? What are the key considerations for developing a secure and user-friendly interface? How do we ensure the safety of user funds and transactions? Are there any specific frameworks or libraries that you recommend for the backend and frontend development? Additionally, what are the challenges commonly faced during the development process and how do you overcome them? I'm particularly interested in hearing about your approach to handling private keys and encryption techniques.