If an app doesn't come with a license, and the code has not been obfuscated, does that mean that the app code and resources are free and open source?
Without a license, you cannot even legally download use a program, much less modify or re-distribute it. Occasionally authors fail to include a license, technically putting users at risk. More often, authors will include a license, and then the conditions grow (can you use it for commercial purposes; can you modify it; can you sell it, or any derivative work). If a person puts software out there "for the taking", e.g. providing a download button, but does not say what license is granted, the courts will probably find that there is an implied license. But that license might be revocable, depending on whether the author received any consideration. The courts would look at "the surrounding circumstances" to determine whether it is reasonable to assume that the author intended to grant permission to e.g. modify (probably, if the source code is attached).
In the United States, all copyrightable works made since 1988, the year of the US accession to the Berne Convention, have been immediately copyrighted upon creation without the need for registration or notice. As an author, there are still many benefits to registration, but works are still fully protected by law without it; the differences mostly relate to damages in infringement suits.
Accordingly, absent a written or implied license you have no rights to use a copyrighted work. If you legitimately obtained it from the author, it likely has an implied right of use, but not of modification and distribution.