How to control the movement of cursor on my iPhone 12 Pro?

Probably you mean insertion point: The insertion point on an iPhone is that blinking vertical line you see whenever you're typing in a text field, email,message, or any other place where you can enter text. It shows you exactly where the next character you type will appear.

There are two ways.

1. Change the Keyboard to Trackpad to move the insertion point (Cursor). Press and Hold the spacebar and move your fingers!
2. 
3. Also, the second option is to touch and slide your finger, watch the gif below
4. 

(1) Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:

• Press and quickly release Volume UP button
• Press and quickly release Volume DOWN button
• Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF). 

(2)  If the above does not work then - Your iPhone have a screen related hardware issue. Contact 👉 Official Apple Support OR you can always get your iPhone examined at the Apple Store 👉 Genius Bar Reservation and Apple Support Options or at an Authorized Service Center for further diagnosis and evaluation.

Axel F.