That was interesting to watch.
YouTube does not recommend the RGB color matrix on uploads but BT.709 (1-1-1).
sRGB TRC will convert to BT.709 TRC.
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QuickTime (AVFoundation & Safari) are a bit of an oddball in that they handle BT.709 tags with inverse camera OETF gamma (2.0-ish), instead of display EOTF gamma (2.4). The “BT.709-A” transform in Resolve attempts to mimic what QuickTime does on an sRGB device, which is roughly Gamma 1.961. Although this trick can be useful for testing, it might not be the best mastering workflow long term.
Many folks have asked Apple about this and not gotten much of an answer: Why does Apple use simplified 1.96… | Apple Developer Forums
Interestingly iOS doesn’t seem to have this issue, and treats BT.709 and Gamma 2.4 tagged files the same.
YouTube (and other platforms) typically remove or reset tags on uploads, meaning it’s up to the browser or video decoder to handle things blind. The last time I tested on macOS I found Safari did the same thing as QuickTime, and Chrome treated most SDR videos as sRGB which was slightly closer to looking correct at least, but still not perfect. This may have changed since last time I tested, and I’m sure it’ll change again in the future.
The common wisdom for web publishing I’ve seen these days is:
-Master footage in the typical SDR way, gamma 2.4 external SDI display in dim surround.
-If targeting the web: transform the already finished footage from 2.4 to 2.2 (pure power)
-Export with typical BT.709 (1-1-1) tags, or basically don’t worry about the tagging.
Although the result isn’t perfect, it tends to be a decent middle ground of both accommodating inconsistent color management (who knows what future software updates will do), as well as compensating for web & mobile viewing environments which tend to be brighter.
Some alternatives to QuickTime I’ve found: IINA (mpv based), or Screen by Video Village.
Meanwhile if you find a way to make the DaVinci UI viewers look correct for SDR mastering, certainly let me know because I’d love to know 
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