Whether you are a experienced developer or beginner, you can use open source mobile apps to modify them and use them.
You can find the top 10 open source mobile apps below :-
Keybase is a messaging app which provides several other exciting features such as file storing, git, webpage etc. This is a open source app created in react native.
source code : https://github.com/keybase/client/tree/master/shared
Telegram is also an online instant messaging and chat app like Whatsapp. But difference is that it is open source.
source code : https://github.com/TGX-Android/Telegram-X.git
Termux provides Linux environment on your pocket mobile.
source code : https://github.com/termux/termux-app.git
I am sure you all know Wikipedia very well. For every knowledge, your are using this and this is also a open-source app.
source code : https://github.com/wikimedia/apps-android-wikipedia.git
OpenKeychain allows you to encrypt message, files etc. Actually, it provides all the cryptographic functions which you can use on any file or message.
source code : https://github.com/open-keychain/open-keychain.git