Why my phone display lining

Lines on the display indicate damage to the signals between the motherboard and the OLED display. Usually it's due to disruptions between the flex cable and the glass the LED circuits are printed on. Unfortunately, that kind of damage isn't repairable, so the only fix is to replace the screen. The recommended method for your phone is to purchase the screen already mounted on the midframe; replacement is basically a process of removing all the internal components from the original phone and transferring them to the new screen+frame assembly. Since iFixit doesn't have a guide to this repair yet, this video will step you through the process.

OnePlus 15 Broken Display Repair 🔧 - Original Screen Replacement Cost & Process - YouTube