The Qt Libraries installed on the development kit are compiled using an Open Source License. You do not need to purchase a license for Open Source projects. If your product requires proprietary source code please contact Reach Support.
You are not required to release your source code. Qt uses the LGPL, which only requires you to release an object version of your program so a customer can (in theory) link against a different library. This is not applicable to the QML code on the module since it is not compiled.
Learn more qt.io/licensing.