What is payload in a frame and where I can find it after capture?

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Generally the payload is the data that's transmitted between the communication partners, excluding the header/meta data.

But it depends on the point of protocol layer. The data of the overlying protocol layers are the payload of that protocol you're looking at.

E.g. if you're looking at a Ethernet frame which transports an HTTP request, the TCP header including the TCP payload is the the IP payload, the HTTP header including the HTTP payload is the TCP payload, and the HTTP request is the HTTP payload.

You can find the payload mostly at the end of that protocol.