It sounds more like you're confusing microSD cards with smartphone internal memory. There is no 'card' in our smartphones, the internal memory storage is a hardware chip soldered to the motherboard (often referred to as 'NAND flash'). Android being built on top of the Linux kernel relies on ext4 as its default file system. Ext4 isn't a file system with a lot of 'bling' but it is stable and reliable.
https://en.wikipedia.org/wiki/Ext4
https://en.wikipedia.org/wiki/Flash_memory
On the other hand we have FAT, Microsoft's 'File Allocation Table'. FAT is an outdated, proprietary, legacy file system Microsoft no longer actively supports. Unlike other, better file systems it was adopted as the default file system for SD cards so it's a curse we're all stuck with. FAT is relatively fragile and easily corruptible so that's one thing to consider when using a FAT-formatted microSD card, it's not the best media for long term archiving of irreplaceable data.
https://en.wikipedia.org/wiki/File_Allocation_Table