how do i transfer files via usb c cable from a pixel 6a (grapheneos) to linux mint?

Mtp is your friend, here is hiw I do it on my devices (samsung a23 and both opensuse and arch):

• I connect my phone to my pc. Then select mtp in the phone notification.

• Start my file manager (dolphine or whatever) and access my phone storage from there.

• Make sure to allow the notification on the phone asking if you want to sahre your storage with the external device.