You cannot use your Lightning headphones with a USB-C port. These interfaces are entirely different - and are not inter-workable.
USB-C Headphones incorporate a DAC - however, you will have a far broader choice of good quality headphones if you choose standard analogue with a 3.5mm audio connector, paired with either Apple’s own USB-C to 3.5mm Headphone Adapter (that incorporates a compatible DAC):
- USB-C to 3.5mm Headphone Jack Adapter
https://store.apple.com/uk/xc/product/MU7E2ZM/A
Some users may choose to use a dedicated third-party DAC to connect their high-quality analogue headphone to the digital USB-C port of the iPad. Here are some examples:
https://www.imore.com/best-dacs-iphone-and-ipad
Instead of a wired headphone (either digital or analogue), many now users prefer wireless Bluetooth headphones or earbuds. Again, there are many to choose from - including Apple’s AirPods, AirPods Pro and AirProds Max.
https://store.apple.com/uk/xc/product/MME73ZM/A
https://store.apple.com/uk/xc/product/MLWK3ZM/A
https://store.apple.com/uk/xc/product/AIRPODSMAX_MAIN