About the moving parts, I am developing a mobile app. I am starting with Swift and hopefully pick up Java for Android later on. I expect heavy data processing

About the moving parts, I am developing a mobile app. I am starting with Swift and hopefully pick up Java for Android later on. I expect heavy data processing at a backend database, maybe on Amazon or Google cloud. I am not sure what I need there. Maybe MySQL? Also do I need something extra for interface between frontend and backend? You have been very helpful. Thanks.