Switch off power backflow on XIAO SAMD21?

[msfujino]

The XIAO_SAMD21 schematic shows a diode, but it may not be implemented.
The following post may be helpful.

Board Supplied 3.3v Power vs USB

What would happen if I supply 3.3v directly to the Xiao on it’s 3v3 pin and then plugin the USBC cable. Wouldn’t that create two separate power supplies on the 3v3 pin? In other words, is the output of the 5v to 3v3 converter compatible with an external 3.3v supply? A simple example of this would be a Xiao acting as an application microcontroller inside another component and someone plugs into the Xiao to do firmware / flash upgrades. At first I thought that it should work, then I wanted to …