I found an app called (TinyTelnet Client )it's an open-source telnet client for android. When I download the source files I only get the java files so I Downloaded the app and decompiled it with an app editor. I think this gave me the manifest and layout xml and res folder files that were not included In the source code. Now my question is. How can I use these files to open the project in android studio so I can modify and run the application ?
You need to ask authors to send you the source code, if it's really an open source project.
Nevertheless, you can find all information about android projects and compiling in official docs: https://developer.android.com/guide/
I bet, it will take a lot of time to build project correctly this way.