Flowable is distributed under the Apache V2 license (http://www.apache.org/licenses/LICENSE-2.0.html).
The Flowable downloads can be found on https://www.flowable.org/downloads.html.
The distribution contains most of the sources as jar files. The source code of Flowable can be found on https://github.com/flowable/flowable-engine.
Flowable runs on a JDK higher than or equal to version 8. Use the JDK packaged with your Linux distribution or go to https://adoptopenjdk.net and click on button "Latest release". There are installation instructions on that page as well. To verify that your installation was successful, run "java -version" on the command line. That should print the installed version of your JDK.
Contributing to Flowable: https://github.com/flowable/flowable-engine/wiki.
Every self-respecting developer should have read this link on how to ask smart questions: http://www.catb.org/~esr/faqs/smart-questions.html.
After you've done that you can post questions and comments on https://forum.flowable.org and create issues in https://github.com/flowable/flowable-engine/issues.