There is a feature in Android called Adoptable storage that does what you described, but not all phone manufacturers support it directly, and it's mostly been phased out over the years for a few reasons.

If you do this that SD card becomes permanently bound to your phone, if you remove it and attach it to a computer it cannot be read anymore, which means if your phone dies that data cannot be recovered,

also most SD cards even the high end ones are drastically slower than the phones built in memory, so if you make the phone treat the SD card as part of the overall system, it will bottleneck the performance of the phone and make it slow and stuttering,

And the way SD cards are made they aren't designed for constant fast read and writes, as that will burn out the card and cause it to fail early

If you drop your phone the SD card might jiggle a little and get disconnected from the for a tiny bit, ordinarily it wouldn't matter but in this Adopted storage mode it would crash your phone and make it reboot.

Some of these reasons is why a lot of app developers force their apps to only be installed on the internal storage and not the SD card. SD cards are best for storing large media and static data like music, movies, photos,

Oh that actually worked fine in Ice Cream Sandwich, they just removed it a bit later so you'd buy a new phone with more storage or something.

Not "196gb of memory" of course, that'd be stupid, it's removable storage and you can't put system files on it for example. But you could easily transfer apps to SD card, they'd just stop working when you changed cards.