There is no manual mode in Camera app

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Solution

By choosing the "Photo Pro" mode, you will find the camera controls which are usually found in professional cameras. You can choose your preferred settings from "Auto", "P mode", "S mode", or "M mode", or "MR Mode" with an interface inspired by Alpha cameras.
NOTE: Xperia phone doesn't have the "A mode" (Aperture Priority). 
NOTE: "MR mode" is only available in Xperia 5 II Xperia 1 III & Xperia 5 III.