Multiple partitions on sdcard (fat, exfat) on Android?
By default, Android only uses the first partition found on the card and mounts it as described in your question. Subsequent partitions will be ignored. That's what's utilized by tools like link2sd: as the system won't interfere on other partitions, they use those for their purposes – be it moving apps to free up space on internal storage (Link2SD), having a swap partition/file, or something else.

Now, if you want to use multiple partitions for your own purposes, you'll have to care for them yourself – which, next to the obvious creation, also includes mounting. To do so, you will most likely need root; I have not read about a user-accessible FUSE method for this (open to learn if it exists :)