Por curiosidade achei no GitHub dele, ele utilizou o local storage para armazenar os dados.

const storedBills = localStorage.getItem('bills');

https://github.com/keidsondesigner/angular-ionic-app-ja-pagou/blob/9230c43c2774abe47b8b82dff0e2d82726df0562/src/app/services/bill.service.ts#L19C27-L19C39