Why doesn't OpenWRT One have an SD card slot? If it has an SD card slot, can it be configured to boot into the system by default using SPI NAND and mount the SD card as a simple storage medium? So how to write the dts of uboot and openwrt? Is there any similar demo reference on the Internet?
On the one hand, there is a PCIe M.2 slot for a NVMe SSD, on the other hand it should be quite straight forward to add a sd card module via mikrobus, the signals are there and even a cheap card module and a few dupont cables might do the trick (on the hardware side) already.
In terms of software, how to write dts? I bought an sd card module and a Dupont cable. I want to try it, but I don’t know how to code.
I don’t think you need to be focused on modifying the dts. OO has the mikroBUS incorporated with the box, and OpenWrt has support for (e)MMC/SD.
Most MMC/SD modules typically use SPI Standards compliant communications (your specific device module should provide this information). I am thinking it should just be a matter of mating your DuPont jumpers from the respective module to the MikroBUS (SCK-Serial Clock, MISO-Master In Slave Out, MOSI-Master Out Slave In, and CS-Chip Select, VCC-3.3V, and GND-GND).
Then you have to set up the OO to configure the OpenWrt firmware to recognize and use the MMC/SD card module. Typically, enabling the MMC/SD interface, installing the drivers and/or modules for the MMC/SD card, and configuring the SD card as a storage device.