Rx/tx queues in Ethernet controllers contain DMA descriptors.[source] DMA descriptors are data structures that specify which DMA (Direct Memory Access) transfer should be performed, and are usually created by the driver stored on the main memory of the host as linked list.[source]
An example userspace network driver with 1000 lines of code is ixy from TU Munich, which can be inspected to understand network drivers better, as well as the accompanying paper.