QoS differences layer 2 and 3

I am having a hard time understanding the concept to apply the policies in my network.

For example in layer 2 I still dont understand is the COS table the same as in the 802.1p protocol?

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And layer 3 DSCP is more confusing. Can I have a real life example for VOIP for example how to create the policy and mark the traffic and then apply it.