Mobile Exercise App

I want to build  mobile app (iOS and Android)
containing several fitness exercises. For each exercise the users will be able to
watch a video that explains them how to make the exercise.
The apps will be free and will have a few fitness exercises included.
Additionally
they will have an option to let users pay a fee and receive the premium
upgrade, where all the content will be available to download.

The mobile app will be in English and will have the following screens:
1. About app. This description will be received from client.

2. List of fitness techniques. This list can contain only the free techniques or
all of them in case the premium upgrade was done. In case the user didn’t
pay for all the techniques, he/she will be able to see that from this screen and
opt to buy the rest of the techniques.

3. Fitness technique details screen will have the name and description text
for the fitness technique and the possibility to play a video.
4. Download files. The app will be small in file size and only if the user pays
for the full functionality, it will download the rest of the files.
5. Payment. The payment will be done using In App Payments features from
Apple and Google.
6. Remembering a payment. In case the user already purchased the full
feature app, and buys a new phone, he should be able to download the
content for free, if he paid once.