Looking to buy an App. Do all Apps come with Source Code? If not, how am I supposed to customize it, edit it, re-label it, synchonize with my backend and add more features on it?
Yes, all items on CodeCanyon include the original source code so that you can customize them.
You can also check the right side of each item’s page to see what kind of files are included. This will often tell you the language(s) that were used. For example, a basic Android app built with Java shows this: