Praise be to Allah.
Striving to spread knowledge and the Qur’an is a great good deed, and making use of modern technology to do that is one of the best and greatest good deeds, if a person does it seeking to please Allah, especially nowadays when followers of whims and desires and the people of kufr, hypocrisy and division are hastening to spread evil, immorality, shirk and kufr, and everything that angers Allah, may He be exalted, via electronic means by using the highest levels of modern technology that mankind has attained.
Making these Islamic apps that contain nothing that is contrary to the teaching of Islam, rather they are aimed at spreading the religion of Allah, and teaching Qur’an and hadeeth, is a great and blessed good deed, if you do it sincerely for the sake of Allah and with the right intention, whilst striving to do your work well and referring it to a committee of specialists in the field in which you are making the app (Qur’an, hadeeth, and so on), for them to check it from an academic point of view and to make sure that it is free of errors.
There is also nothing wrong with you selling it to people via this electronic seller (the App Store).
For more information please see the answer to question no. 171039
And Allah knows best.