Hi,
Regarding the microSD issue, I think there are some misunderstanding here.
There are two kinds of jetson nano modules. One is sdcard module with sdcard slot on the module.
Another one is emmc module which does not have sdcard slot but only emmc.
The website which tells you that you can flash a “sdcard image” on Windows machine is basically for sdcard module. Not for emmc module.
Then the next problem is, then why you have a sdcard slot there. That is the uncertain part. I guess it is probably on the carrier board itself. And how that carrier board is coming from is not what I can answer.
If that carrier board is made by you, then we can guide you how to modify kernel and device tree to enable that sdcard slot.
However, if that carrier board is just bought by you and you have no other info about it. For example, schematic or driver code, then we are not able to guide you what to do. You need to find the board vendor for help. Generally they already got help from us and enable that sdcard slot with their BSP.
You need to contact them for that BSP.