Question about ASUS GeForce ROG STRIX GTX1080TI O11G GAMING

The corresponding cables are normally part of the scope of delivery of a power supply unit.

They are included with the power supply unit. Never use cables that were not included with the power supply unit. Cables from other models or even from the same model (e.g. one version 2015, one version 2018) are usually incompatible, even if they look the same. If you use cables from other manufacturers, it is possible that you will completely destroy your power supply and graphics card when you switch on the PC.