MacBook – Connecting to MacBook Air 2017 with usb-c connection

The MBA GPU may be limited to a single external monitor, but the USB to HDMI requires a driver installed on the MBA and that video/image is not using the GPU, it's using the CPU. So the limit is bypassed. Now it might be more complicated than this, but I think is this the basic premise. Be aware that for fast moving videos or games the USB connected display may disappoint. Also if the MBA is running near 100% CPU usage and then you add this USB display it may also disappoint. If you looked to add more screen for email, YouTube it's likely to be fine. The hub might be more of an issue, if the cable is USB 3 make sure the hubs too

The reason that when you plug in your (known to be good/working) USB-C to HDMI video adapters into USB-C hubs is because the USB-C hubs don't pass the video signal that originates from the MacBook Thunderbolt port.

If you need a hub for connectivity and you want to have HDMI output, you either have to get a dock or a hub that specifically passes the video signals like the Anker Premium USB-C Hub with HDMI and Power Delivery.

Apple tried to make everything simpler with having one type of port, but (IMO) they made it more confusing. A few things to keep in mind:

• USB-C is the physical port type. USB 3.1 is the USB specification that sets the standard for how the port functions
• Thunderbolt 3 is the actual port on your MacBook. It just happens to use USB-C.
• Thunderbolt 3 has many functions built into one connector
  • Video (HDMI 2.0 and DisplayPort 1.2)
  • Power Delivery
  • USB (3.1 Gen 2)
  • PCIe (PCI Express 3.0)

That's just a quick summary; for full details, see Thunderbolt 3.

So, just because an adapter "says" USB-C, dosen't mean it's going to carry all those signals/protocols. USB-C is often gives the end user the perception that "if it fits, it works." Unfortunately, now you have to verify that that what you want to do is supported by the adapter.