Here are some free code of android app you can download or implement.
NewPipe is developed by TeamNewPipe. To create your own youtube app you can use this s
ource code. Newpipe is light weight app. This app does not use any youtube library. It parses only website data. So this app can be run without google play services.
LeafPic is developbed by HoraApps under GPLv3 license. This is an alternative gallery app for android. It totally ad free and contains every features of stock gallery app. On top of that LeafPic followed material design.
AmazeFileManager is developed by TeamAmaze. It an awesome file browser app you can implement. AmazeFileManager has material design. This app provides compressing features beside of normal cu, copy and delete features. It has also AES Encryption and Decryption ability.
Note taking application can be a great choice for beginner. Omni-Notes is developed by federicoiosue. It has simple interface and smart behavior. Omni-Notes contains material design, file attachment system, to do list, sketch note mode, multiple widget and top of that it supports 30+ languages.
its my favourite one. Tiber is a material design music player developed by naman14. It n awesome looking music player which supports lyrics on the go. Timber supports android wear, android auto. It has also LastFM scrobble ability.
Qksms is an alternative of stock sms app in andoroid which is developed by moezbhatti. You can develop it further if you want.
MovieGuide is awesome looking movie info app. It uses the TMDb api to get the data of movies. This app developed by esoxjem. MoveiGuide is an example of MVP pattern, RxJava, Dagger 2 and Uncle Bob Martin’s Clean Architecture approach.
CoCoin is developed by Nightonke. Its an finance management app. Nice UI and multiple accounting application. People can do the accounting job easily in this app with a password protection on.