"...For anyone out there wants to practice any aspects of their CW copying, Ditto CW Player is a free app that I can recommend. I have no connection with the app or the developer so not trying to sell it (it is free anyway).

Billy WB1LLY created what amounts to an app version of Morse Code Ninja (which I think is fully open source) that is very convenient to use when out and about. There is no internet connection needed to use the app. Another plus is that Morse Code Ninja materials are limited to 50 wpm, but this app is not.

Support is excellent. The app was initially slightly chopping off the first part of the first character (on my phone at least), and I mentioned it to Billy.
This amounted to losing the first dit at 50 wpm. Within 24 hours he added a slight delay at the start and now it sounds perfect to me!..."

https://play.google.com/store/apps/details?id=com.billyfrancisco.dittocw&pli=1