WebSocketSharp .onMessage on main thread? / handling in Unity?

Question

I find it very difficult to find a simple reliable library to use, in Unity, for a simple websocket client. WebSocketSharp (ie, https://github.com/sta/websocket-sharp ) seems fine but many items are unexplained.

When incoming messages arrive,

    wssharp_connection.OnMessage += (sender, e) =>
    {
        if (!e.IsText) { return; }
        Debug.Log(">> OnMessage, " + e.Data);
        Handle(e.Data);
        // but wait, not the main Unity thread??
    };

   void Handle(string msg)
   {
   .. your conventional Unity call, in a monobehavior
   }

Testing shows it seems to arrive on (always? sometimes?) not the main Unity thread.

Does anyone know

1. In fact is it always not the main thread, or can it vary?

2. If always another thread, in fact is it always the same one thread, or does it vary/many?

Since every single Unity app that uses a websocket client has to do this, I'm surprised that there are not 100s of discussion about how to handle incoming messages, including on the WebSocketSharp page,

but here's what I would do, just in the usual way you handle "threading" in a frame-based engine like Unity:

    wssharp_connection.OnMessage += (sender, e) =>
    {
        if (!e.IsText) { return; }
        incoming_messages.Enqueue(e.Data);
    };

// and ...

ConcurrentQueue<string> incoming_messages = new ConcurrentQueue<string>();
void Update()
{
    if (incoming_messages.TryDequeue(out var message))
    {
        Handle(message);
    }
}

So then

3. Is that indeed the usual way to handle the threading problem in WebSocketSharp, when using inside Unity?

Given how many billion Unity apps there are with websockets clients, this must be a pretty well-trodden path.