Realtek does contribute directly to the kernel, but I think it depends on which internal team is handling the device support. For their PCIe 802.11ac and 802.11ax wifi cards Realtek devs wrote and upstreamed rtw88[1] and rtw89[2] respectively.
Wasn't rtw88 supposed to abstract away the transport so it could be used with PCIe, USB and SDIO? [0]
The later two never materialized (there is the out of tree rtw88-usb [1] but it's not very active and not maintained by Realtek)
[0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...
[1] https://github.com/ulli-kroll/rtw88-usb