Does iTunes Connect has an API? How do some applications download financial and sale reports to computer? Are there some C/Objective-C API wrappers?
On November/18, Apple finally released the Appstore Connect REST API. Referencing the Apple description:
TestFlight. Manage beta builds of your app, testers, and groups.
Users and Access. Send invitations for users to join your team. Adjust their level of access or remove users.
Reporting. Download sales and financial reports.
Good news on this one: on 2018 WWDC, Apple announced the Appstore Connect API - where you'll be able to access almost everything related to your Appstore Connect management. You can see two related talks here and here.
The only curious thing is that its release date was supposed to be late this summer - so it's probably delayed. Nonetheless we should be able to see a release soon.
Official Reporter tool from Apple
https://help.apple.com/itc/appsreporterguide/#/
iTunes finally released an auto download tool as noted in the PDF
http://www.apple.com/itunesnews/docs/AppStoreReportingInstructions.pdf
Here is the class file
http://www.apple.com/itunesnews/docs/Autoingestion.class.zip