@Pankajk5 As I can see from the discussions above you seem to be having trust issues with third party apps and it's completely normal as I used to have same but then I learned that if third party apps are taken from offical website and @andrewilleyif you are into android development then you can debug the apk and see its AndroidManifest.xml to see which permission it is requesting. So as for a video streaming app such as YouTube Vanced and Twitch there shouldn't be many permissions. The basic permissions can be to request your device storage to download videos on your device for offline viewing. For eg: if you write this in your AndroidManifest file
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
the app will ask for Read and Write permission of your device. As for safety of YouTube Vanced I debuged the apk and looked into it's AndroidManifest file and couldn't find any nasty permission, so YouTube Vanced is safe. More details here