Drives blindly store and retrieve blocks wherever you tell them, with no awareness of how or if they relate to one another. It's a filesystem's job to keep track of what's where. Filesystems get fragmented over time, and especially as they get full. The more full they get, the more seeking and shuffling they have to do to find a place to write stuff. This will be the case even after the last spinning drive rusts out, as even flash eventually has to contend with fragmentation. Heck, even RAM has to deal with fragmentation. See the discussion from the last few weeks about the ongoing work to figure out a contiguous memory allocator in Linux. It's one of the great unsolved problems in general comparing that you and your descendants would be set for life if you could solve.
Not quite, AFAIK? Drive controllers may internally remap blocks to physical disk blocks (e.g. when a bad sector is detected; see the SMART attribute Reallocated Sector Count).
It makes more sense but it's not true for the modern CoW filesystems that I'm familiar with. Those allocate free space in slabs that they write to sequentially.
