The OnePlus 9 5G is a mid-range smartphone, released in March, 2021. It's the successor of the OnePlus 8 and features a 6.55" 120 Hz Fluid AMOLED and runs on a Qualcomm SM8350 Snapdragon 888 5G chipset.
The digitizer screen has cracked internally. It happens when the phone falls. If it falls again you might find even more lines until it's impossible to read what's displayed.