Nossiac has built mediatek drivers (similar to official), with some differences. They can be driven by some config files and IOCTL calls. They are not compatible with cfg80211/mac80211 subsystem, that's why they need custom scripts on OpenWRT.
Some of these sources leaked, you can find them on github. I am not sure about license status.
Almost all of official Mediatek drivers do not support cfg80211, but some of them have included headers and source code referencing to cfg80211 subsystem. As cfg80211 subsystem changes often, some effort is needed (especially with mt7603 driver) to successfully compile.
Anyway enabling this source code (flags in Kconfig), makes driver dependent of cfg80211 kernel module and allows hardware to be controlled by cfg80211 subsystem.
In theory, OpenWRT fully works with such drivers (one example is brcmfmac, other is mwlwifi). mt7603 driver registers as a FullMAC driver, with mac actually implemented inside driver. mt7615 driver registers as a FullMAC driver, but I am not sure how much is done on hardware/firmware.
First, I want to check if the driver works - that's why I uploaded my compilation. If it works, I could share a source code. If not, I will know that the source code is useless (and could explain why Mediatek never included support for cfg80211).
Drivers used in OpenWRT are written from scratch. I see that nbd168 even tried to modify official driver (mt76x2), there is a repo here https://github.com/openwrt/mtk-wifi-gpl/commits/master .
In theory these drivers (https://github.com/openwrt/mt76) have much better source code, are less complicated and developed for many years. But they are also bugged, and developers prefer to include crappy support for newer devices than provide at least one stable hardware support. Even MT7612 chipsets have problems with mt76 driver, while it is being developed for more than 5 years - just look https://github.com/openwrt/mt76/issues here.