Do I have to use their technology? Can I not use my own?
If you plan on developing your own App (be it mobile or web), you will then have to use their API. Everything is provided for by the solution provider, so that they manage the compliance, payment processing & settlement, transaction management, ID/KYC, etc. You simply have to use their API and develop your UI/UX on top of it.
In other options, you can use their white-label website (which is a responsive website, i.e. will work on the mobile as well as on tablet). You can also opt to use their white-label mobile app only. Or you can use both. It all depends on what you are comfortable with and what you opt for.
The pricing for these four options can be seen here.