Filesystem is the layer of software that turns a raw disk into files and folders
Disk offers something dumb, a huge numbered array of fixed-size blocks. Block 0, block 1, block 2, up to a few billion
You can only read or write to blocks
Filesystems (ext4, APFS, NTFS, XFS, etc.) is code that imposes structure on that array
It maintains which blocks are free and which blocks belong to which file