Let me preface this by saying I am not a lawyer, but I am an inventor with several patents including algorithm based patents. As always, I highly recommend you work with an actual patent attorney. This is especially true with software related patents. A incompetently drafted and prosecuted patent may provide little or no actual protection.
is it recommended to file multiple patent applications for each
individual novel feature I developed; or if I end up filing 1
provisional application for my overall app, then would each novel
feature be protected, as well as my overall concept for the app?
Please understand that the filing of a provisional patent application does not provide any patent protection by itself. All it does is potentially provide an earlier priority date. You will still need to file non-provisional patent applications and have them grant to get patent protection. It may be adequate to file a single provisional covering every potentially novel idea in your app, but I'd expect that you will need to file several non-provisionals as the USPTO generally only wants one invention per patent.
if I file a patent for my app which has already existing features from
other apps, but also has new inventions/technologies that I developed
therefore they qualify as novel, then is it still possible that my
whole application would be rejected because of the non novel features?
You can file for patent protection for an extension of a pre-existing feature or invention. The extension must qualify for a patent. However, this new feature patent only grants you the ability to keep someone else from using the feature. It doesn't guarantee that you have freedom to operate. This means if the existing feature is patented, you need to obtain a license to use it.
can I file multiple provisionals, 1 for my app and 1 for each feature
at the same time, and if so then does that even make sense to do?
You absolutely can, but whether it makes sense or not is really a question for a patent attorney or agent. It depends on the specifics of your invention and you can't disclose that publicly before filing. Thus, there is no way for people answering questions on this site to provide an authoritative answer.
if I have a feature that uses calculations provided by another
company's technology and displays them in a novel way, then does that
automatically disqualifies that feature as non novel, even if the
functionality of the feature itself is unique?
As I already answered, you may be able to get a patent on the new feature if it is novel, useful and non-obvious. This doesn't mean you can use the feature if you you can't get a license to use the other companies technology.
