USB 3.0 (Type A) to USB-C on Early 2015 MacBook Pro for HDMI
USB-C is indeed just a connector, and so a number of different standards can use it. HDMI output needs the Thunderbolt 3 standard to be delivered over the USB-C connectors, but your 2015 MacBook Pro doesn't and can't use that standard on its USB-A ports. A USB-A to USB-C adaptor will simply pass on the USB data.