SDcard from Thonny: not clear

[mastraa]

Dear all,

I'm quite new in Micropython and Pico board.
Today I add an sd card to my project, I need it not to fill internal memory with bid data files.

Surprisingly I saw a file with same name of my sd-file from Thonny file-column-tree. Is that correct? I should have access to sd file from Thonny when Pico is connected.
It seems to be like this because if i remove sd card I cannot see it anymore, but I didn't think it was possible so I have some reflection on that.

Furthermore: may you explain me what this function needs for?

Mount filesystem

vfs = uos.VfsFat(sd)
uos.mount(vfs, "/sd")

Thank you all,
Andrea