The memory card, or the "large built-in memory partition" (which has been replacing a real memory card on the newer devices... since about Galaxy Nexus, end of 2011) --
-- seems like a natural place for a backup file.
Here you can easily copy it somewhere with a file manager, or via an USB cable, that sort of thing.
Using internal memory for backups means the file won't be visible to the user, so not as much use. Internal memory is also erased when you do a device reset, which makes backing up there even less useful.
Now, if your phone has a large built-in storage *and* a real microSd slot which is empty -- then you should be able to make backups. The large built-in storage is treated by Android as "the" memory card.
The path is usually /mnt/sdcard or /storage/emulated/0