The XAIO BLE Sense, has two UARTS that software can access: Serial and Serial1.
The Serial port is connected through the USB connector, and appears as a USB CDC object on Windows or Linux workstations (and, I assume, on Macs but I have no way of testing).
The Serial1 port connections are logic-level signals on the XAIO BLE pins 06 and 07.
I just posted a little test program for XIAO. The exact code also works on the XIAO BLE. (Well, I changed a comment, but otherwise, the code and the functionality are identical.)
Yes, really. Serial1 is alive and well on XIAO and XIAO BLE Sense (and other XIAO stuff as well if anyone wants to give it a try).
Regards,
Dave