We do not suggest for using the debug UART port for custom usage.
(i.e. please do not connect it to your custom MCU because it will still output log after boot up)
You have to check the full dmesg and device tree to confirm which node is generated for UART0(3110000.serial).
UART0 is from M.2 Key E rather than 40-pins expansion header.
Please do not use debug UART port for your custom serial device (i.e. MCU) because you will get unexpected log output from several firmwares.