In chrome web inspector open the network tab and click on the ws tab. Then click on the connection to see the message traffic (if you don’t see the connection just reload). You then need to click the message tab. The messages are in binary, so you need to click on a message to see the dump of the payload.
ms doesn’t document the message formats, so you will need to look at the source code to understand. In general the client sends events to the server and the server send razor render tree updates to the client.
https://github.com/dotnet/aspnetcore/tree/main/src/Components/Web.JS