How does the iOS app connect to cloud providers (OneDrive, etc)?

I’m just getting started with Cryptomator and at first glance it appears to fit my use cases nicely. If I determine that it fits my needs fully I will definitely contribute. I’ve installed the Windows version and I like what I see. I’ve also purchased the iOS version and I have a question about how it connects to cloud services. It’s asking for my OneDrive credentials. Is the app communicating directly with OneDrive via well-known API’s? Are my OneDrive credentials shared with anyone anywhere besides OneDrive directly?

That’s correct. We’re using the Microsoft Graph SDK (and NXOAuth2 MSAuthenticationProvider for authentication) to communicate with OneDrive.

No. Account information (e.g., tokens so that you don’t have to login again and again) is stored locally on your device and secured in the iOS keychain.