How does USB-C on the MacBook connect to multiple devices? It seems that the new USB-C connector is good for charging, good for HDMI output, good for connecting to new devices that use USB-C connectors, and good for older USB devices like an old scanner, printer, external hard drive, and video capturing box.

I suppose the older devices can be connected by using a USB-C to multi-port USB hub.  However, I wonder how this USB-C connector can be connected to multiple USB-C devices, HDMI output, and older USB devices?  For example, let's say an HDMI output adaptor is plugged into the Macbook, then how do you charge the Macbook, and how can another USB-C device as well as an older USB device be both plugged in?
Although Apple named it USB-C, it's officially called USB Type-C.

USB Type-C does not allow daisy chaining like Thunderbolt or FireWire.
That being said, you will need a USB Type-C hub.

USB Type-C Hub

A USB Type-C hub will likely have:


One USB Type-C DFP (Downstream Facing Port) connecting to the host.
Multiple USB Type-C UFP (Upstream Facing Ports) connecting to devices or another hub.


or


Multiple USB Type-A UFP (Upstream Facing Ports) connecting to devices or another hub (USB Type-A are the old USB ports).


USB Type-C hubs will need to comply with the USB 3.1 specifications. USB 3.1 provides backwards compatibility to older USB standards, therefore you would be able to add older USB devices with the use of an adapter or if the hub provides older UBS Type-A ports.

Example: USB Type-C hub

With a USB Type-C hub, you could add multiple USB Type-C devices, for example 2 USB Type-C monitors, as long as the host can drive/support them. In that sense, USB Type-C hubs work just like 'normal' USB hubs.

Example: USB Type-C devices

LaCie announced a USB Type-C external hard drive. If you add this to a laptop with only 1 USB Type-C port (and this laptop also charges via that same USB Type-C port, like the MacBook 12" mid-2015), then you cannot use the hard drive and charge the laptop simultaneously, unless you add a USB Type-C hub (that allows for charging) between the laptop and the hard drive.

Example: USB Type-A devices

You could add a USB Type-C to USB Type-A hub or adapter. For example an Apple USB-C to USB, USB-C to HDMI or USB-C to VGA adapter. Thereby making old USB devices, like a keyboard or a scanner, available to your host.

Note that using a USB-C to HDMI or USB-C to VGA adapter you can simultaneously:


Charge your MacBook
Use HDMI/VGA
Use an old USB Type-A device (this can also be a USB Type-A hub with multiple devices).


USB 3.1 specifications

Please read the USB specifications, including USB Type-C, for more details: http://www.usb.org/developers/docs/