They can because they essentially support Google chipsets, which are not blobby like MediaTek or Qualcomm because Google for all its faults is still relatively open (except their recent change in release schedules is why the Pixel 10 series still only has experimental GrapheneOS support).
>>vbezhe+r8
No, but they used to publish the source code for the drivers as part of AOSP. Now they no longer publish the device trees. Check out GrapheneOS' other Mastodon posts for the gory details.