I have recently invested in a HDMI touchscreen from Amazon, and a Raspberry Pi 4, For the touchscreen to work it must be connected via GPIO, however my pi cannot supply enough power to it from my usb power bank which can provide enough power for the pi but not the display. As the display has a usb port for power as well as GPIO, I attempted powering the display from usb, however this caused the pi to boot up, hinting that there is some connection causing power to go into the pi from GPIO, thus I question whether it is safe to power the pi from GPIO and USB-C simultaneously?
Update (6/05/20): I am asking this question as when I power it from the USB-C on the Pi alone it gives me a yellow lightning bolt in the top right-hand corner.
How to connect Rpi to a HDMI Touch LCD Display?
HDMI Touch Screen Wiring Diagram
Appendix A – XPT2046 Touch Screen Controller Block Diagram and Operation
Appendix B – LCD Display Selection Guide
End of Answer