/mnt/sd is what's called the 'mount point'. It's just an ordinary directory until the OS starts routing communication with the SD card through the filesystem through that filepath.
In a regular unix/linux system, the command to associate a storage device with a directory in the filesystem is called `mount`. It has a variety of specialized sub-programs like `mount-msdos`, `mount-ext2`, `mount-nfs`, etc. Each one takes a 'device file' (a piece of software running in the kernel that's presented as if it were a file in the /dev directory) and the name of a directory that will serve as the mount point. The device file actually handles all the IO between the kernel and the storage device. The part of the kernel that handles the filesystem associates any filepath starting with the path to the mount point as a command to communicate with the external device.
Generally device files in /dev follow conventional names like /dev/disk0, /dev/disk1, with each category of name being associated with a specific kind of IO hardware.
Embedded versions of Linux have a habit of taking all the different programs that manage the OS behind the scenes and combining their features into a single swiss-army-knife of a program. You can argue that such programs are more compact, but unix has a long-standing design philosophy that points exactly the opposite way.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.