I've been looking at the installation/upgrade notes for Linux Mint 20.2, and they're making an issue about merging /bin and /usr/bin, etc. Apparently Linuxes in general are moving away from that separation, and it has been a mystery and point of confusion (to me at least) why they are there.
Well, reading up the background, it seems to be simply that the origins of Unix was when disk packs were very small, and when they ran out of space on the first drive they had to start putting stuff not critical to the initial boot on the next (/usr) drive (and then when that was full too, the /home drive)!
Apparently, everything will now be in /usr/bin (etc), with symlinks from /bin to /usr/bin (etc), so it will no longer matter where some script thinks an executable should be – all roads lead to Rome (or /usr/bin), which (apparently) will make the different Linux distributions rather more compatible with each other than they have been (making porting much easier).
Why that way around and not put everything in /bin (as per the original Unix) is something else to get my head around.