How does it work? Can I use an Android app to send money? Do I just enter the recipient's mobile phone number or is there something else that identifies the recipient?
For plain IMPS, you need to use your bank’s App. For UPI, you can use any app eg google tez, or WhatsApp. One requirement for both is you need to link mobile number with your bank account and for plain IMPS you need additional id called MMID for transfer. UPI does not need it. It also has cool feature of requesting money from other UPI addressses.
Yep, the mobile phone number or something called as Virtual Payment Address that looks like an email address (name@bankname). It's actually quite useful and a lot of apps support it including WhatsApp and Google's Tez.
The use of mobile numbers to instantly transfer money is using UPI, a system built on top of IMPS. UPI is actually just an API. Third-party apps provide payment features on top of UPI.