Hi there,
It’s the Board to Board connector (hence B2B) is what they call it, basically it’s the expansion connector thats on the Sense version. , The Xiao ESP32S3 doesn’t have it unless you order the Sense version with the camera and SD daughter board. (would need a new SKU#) and reasonable price.
It’s WAY better solution than the ridiculous Idea (IMO) 
of putting more inaccessible pins and pads on the bottom of the Xiao and requiring MORE soldering. BAD IDEA! The "MORE I/O " 
board just plugs in and go. No messing around and fumbling the soldering iron like on the live stream. 
I can put pins or solder to the pads way easier without any possibility of damaging the MCU or the associated heat unsoldering some unknown component on the top side of the board.
Try DeLidding a Xiao and then apply heat on the bottom (let say too much heat) from too large of a soldering iron, watch the top side components the possibility of developing cold solder joints on some of those .201 or .403 SMT size resistors for example doesn’t take much.
The connector is WAY more reliable.
HTH
GL 
PJ
you could do any number of connectors or types after that.
like these.
