I want to make some changes to GitHub - stefan-niedermann/nextcloud-deck: 📋 Android client for nextcloud deck app. First, I built the Deck app to see how it works. But it requires NextCloud Files. So I have to install Nextcloud Android and type in my server IP. Is there anyway to access my server by using Deck, not via Nextcloud Android?
No, the Deck Android app completely relies on the Nextcloud Single Sign On library. A few reasons why we are doing this:
This allows us to reuse the whole network stack without the need to handle self signed certificates, special server setups and other edge cases in infrastructure and servers 
In terms of security, we don’t have to ask for, know or store any credentials. We can’t lose what we don’t have 
The user experience is superior: Just select your existing account in the best case. No need to enter the URL and credentials over and over again. Same flow and UI in multiple third party apps like News, Notes, Tables, … 
The only drawback is, that you need the Nextcloud Files app being installed. Same as for example for Google, where you have installed centralized Google Play Services on your phone (and I think it is safe to assume that most people have installed Nextcloud Files anyway).
We currently have no plans to support a login without Single Sign On.