Hello,
Can we use github code and upload custom application with modified UI and additional features?
I don’t really get your question.
if your question is about downloading the source code of some apps from Github, modify it and use your own version of said app on your Nextcloud; Yes you can, you can do that and this is one of the main purpose of free software/open source 
You can also share your modification on your own github, or send your modification to the original github and they might be accepted and included in next releases of the app.
You will have more informations regarding this subject: https://www.gnu.org/licenses/agpl-3.0.html
As one of the devs of android, yes you can.
But you will have to provide the source code of your changes in some public place.
And to avoid splitting up the (always) limited men power, maybe we can work together / you can contribute your enhancements back to “us”?
If you want so, just open up some issues and tell us what is missing.