I'm planning to create a Arduino device that measures distance, and sends the data to AWS IOT via WIFI . After that i want to see the measurements on an Android or iphone App. 

How can i make my idea to work with lots of users? I mean, lets say I sold 100 devices, how each buyer will connect to his own device?

I'm thinking that each Arduino will have a Unique ID/UUID, but how do I make that with AWS IOT? Should I use other server/MQTT platform?

I'm using Arduino ESP32.