Modern deep packet identification gear looks at packet size and pacing to distinguish video traffic patterns. It is possible to fool these systems, but it is not trivial.
dunno about netflix, but youtube usually does. They send you a chunk of data as fast as possible, and then send nothing till your client requests the next chunk.
By doing that, they can get a good estimate of your connections available bandwidth, which is needed for the decision whether to automatically switch to a higher/lower quality feed.
It also means they can use 'dumb' servers which don't do any application-specific logic for throttling.
Recently I've noticed my phone disconnecting from my Wi-Fi router (a relatively underpowered device running OpenWRT and WireGuard) when starting YouTube videos. I guess this could explain it, if the router is hiccuping on that initial blast of data. I also use a Safari extension that lets me choose the YouTube video quality, so that might affect it if I'm not giving the feedback they expect for "auto" quality selection.
(The issue is not the router, though - this doesn't happen on my laptop. I think my phone is just more aggressive in assuming the Wi-Fi is down and it's better to switch to cellular. But couple that with my phone's policy to use VPN on cellular, and the switch becomes much higher friction. I tried simply disabling cellular data, but then it's even worse because every time the phone disconnects from Wi-Fi it pops an alert telling me to enable cellular data.)
