What the part is

The “motherboard flex cable” (also called “flex cable ribbon”, “flex ribbon”, or “motherboard flex ribbon”) is a thin flexible printed circuit (FPC) that connects different modules of the phone — for example the motherboard/mainboard to other components like ports, buttons, sensors, etc.

In this case it’s a specific replacement part listed for the OnePlus 7T. For example: a listing describes it as “Motherboard flex cable ribbon replacement for OnePlus 7T” from a spare-parts supplier. Mobspares+2eBay+2

Typically the part is essential when the original cable is damaged (torn, cracked), or a module stops working (charging port, sensor, etc) and is traced back to a flex/cable issue.