Is there a device that allows two or more connected USB devices to communicate, yet their power be supplied by an external source (such as a USB power bank or charger).
I tried to make this illustration to clarify what I mean.
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USB-C hubs with power pass-through are fairly common. Often sold with the Power Delivery (PD) mark followed by the power rating for that device. Here is one such hub that I use regularly to connect peripherals and charge simultaneously. However, this hub does not have a USB-C slave connection.
Hubs that also have a port for a USB-C slave device are more rare. Since you want to have an attached device that is also USB-C, I would recommend this one. You plug your main device (phone/laptop) into male connection, your USB-C device that is the slave (flash memory, etc) into the bottom port, and the charger plugs into the port on the side.
Laptop ---[_Hub_]---- Phone | Charger