What should be done with the unconnected power pins if powered via USB?

- Vin
- 2x 3.3V

I noticed on this usage of a Teensy 4.1 the 3.3V are connected to a 10n capacitor.
https://gitlab.com/CrazzyFrenchDude/..._Schematic.pdf

Also, my PCB has a separate 24V power supply connected to it. The USB GND is connected to the power supply GND back at the power supply. Given that, should I then NOT connect the Teensy GND pins to the PCB GND? Wouldn't that create a ground loop?

Other components on my PCB need more than the 3.3V 250mA the Teensy can supply, so I have a 24V to 3.3V buck converter on the board for those components.