I don't know if this qualifies as 'apps' but there are thousands of sermons by D.Martyn Lloyd-Jones at MLJtrust.org. Bible Hub offers a lot of resources, and Logos, among others, have free stuff if you download their free reading software. This on an android rather than Apple, but I'm sure they are available for one or the other.
Two Ipad apps I have profitted from:
Ireformed reference library: has the catechisms (Westminster large and short, Heidelberg), Creeds (Nicene, Apostles and Athanasian) and Confessions (Westminster, Belgic and Dordt). Has scripture references for the Westminster confession and catechisms, which are helpful as well.
I enjoy the RSB bible, and use this for my primary bible reading (make sure to look at the different translations you can download with it.
On android ,I'm sure it' on apple also, I have an ap called quizification. ..I think..that randomly asks catechism questions and let' you grade yourself. I have it on my phone so everytime I look at it there is a new question for me to try and answer. I enjoy it.