Can I upload the App in testing mode so that it can't available for normal user and i will change it to public mode later on. please help how to do so
Sure, you can either invite people as internal testers/team members - has a limit of 25 testers - and send them apps via testflight over app store connect, or you can upload your build and submit for review for beta testing, and then invite whoever you want as external testers - has a limit of 10.000 testers -. For detailed info