SignalR: Clarify connection vs client

Let me explain my point of view on many aspects of SignalR.
I have lots of experience but very little with ASP.NET (so I could be wrong)
It's all about AspNetCore (new one) SignalR.
1 - SignalR lib use "connection" and "client" at different places but both seems to represent the same thing. Why having 2 differents terms for the same things? If it is not the case, what is the difference and make it clear in the documentation.
2 - SignalR use Hub to send notification. To access Hub from a worker thread, we should use IHubContext. But IHubContext needs to have a Context to access it (from the documentation). But nowhere, it is written how to get a Context from a worker thread, why?
3 - Related to question 2, Why there is no Hub builder object to easily and quickly constract a Hub without all that complexity. Or another easy way to get it?
4 - Calling a Hub does not enable the user to iterate over Client (or connection)? Why being so restrictive? How a programmer can set client (connection) specific filter (notification type filter) if there is nothing in the framework that enable it to works with it? Wouldn't be a too much restrictive design?
Ref: https://github.com/dotnet/aspnetcore/issues/47749
Note: Could anybody explain why there is AspNet.core and AspNetCore ??? That's hard to follow and find difference in documentation, issues, stackoverflow and everywhere. Very confusing!