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When I was looking into the use of CoS (Class of Service) bits, I came across a lot of articles which said that it was used to mark different types of traffic and offer varying quality of service. For example, https://www.techtarget.com/searchnetworking/definition/Class-of-Service-CoS Most ISPs in the USA also throttle video traffic to 480p.
How do these ISPs detect which type of traffic a particular packet is carrying? I'm assuming they cannot read it's contents as they would be encrypted.
As you've read, CoS marks traffic at layer 2. Since layer 2 information is striped off at every router hop, it is only useful on the local network. DSCP (ToS) is a layer 3 marking that follows the packet through its entire journey.
Most ISPs in the USA also throttle video traffic to 480p.
I doubt that's true, but even it it were, there are many ways to identify video traffic. An ISP can look at source or destination addresses/ports. Also note that questions about networks you don't control are off topic here.
