I tried to download the sleep apnea App, SleepCheck, partly to see how easy to use it was , and after paying $7.99 I received a message that it did not work on my phone, which is a current Oppo A54 5G.
They are not going to sell many Apps if they are restricted to a few phones.
If this applies to their other Apps, it may be why they never make any worthwhile cash.