Davinci Resolve's supported Codecs list shows it only supports decoding VP9 if placed in an mov container. Add to that the audio limitations also present on the Linux version and mov is somewhat preferred overall.
The issue of course is Quicktime File Format doesn't officially support VP9.
Attempting to do so with ffmpeg like so
ffmpeg -i input.webm -c:v libvpx-vp9 -c:a pcm_s16le output.mov
results in the error
[mov @ 0x55719f592d40] vp9 only supported in MP4.
I contacted Blackmagic Design (BMD) support a few weeks ago now asking if there was an updated codecs list with the version 16 to 17 update (there isn't), and asking if the mov extension requirement for VP9 was in error. After providing a sample video they replied that VP9 is
... a Quicktime Render Option in Mac which would go in a .mov package.
going on to say that
I have also confirmed that using Adobe Media Encoder along with the Webm plug-in that you can render [your video] as a QuickTime .mov format.
I was able to test the Adobe Media Encoder method myself, but the plug-in simply added an option to manipulate .webm files and export files with the .webm extension using VP9 codec. Webm extension doesn't import into Resolve though.
The Quicktime option in Adobe Media Encoder uses .mov extension as expected, but the codecs aren't related to VP9 as far as I'm aware. I encoded a test video with each available Quicktime options in Adobe Media Encoder, checking each with ffprobe after, and none reported VP9 like BMD seemed to be saying was possible. They do match to other supported codecs listed on the PDF though. I relayed that to BMD to which they replied,
Using the Adobe Encoder encodes the webm vp9 codec to Pro Res 422 which is .mov
It does not specifically rewrap the vp9 codec as .mov
A video in Resolve can be rendered as VP9.mov but there doesn't appear to be any way to import a VP9.webm video into Resolve first.
If I'm understanding Blackmagic Designs response to my reply, then it seems they're confirming that yes, none of those options actually encode a .mov extension file which contains VP9 encoded video. Yet per their supported codec document, across all platforms that is the only extension that is able to import and decode VP9 coded video.
I was also unable to render a video from Resolve as a VP9 coded video in mov format as support stated is possible, testing on both Windows and Linux.
So am I missing something really obvious here? Or is this actually a combination of codecs and container that doesn't exist in a sane way?