I have:
vonc@voncavn7 MINGW64 /usr
$ ls
bin  etc  lib  libexec  share  ssl
If I check usr:
D:\prgs\gits\current\usr>ls 
bin/  etc/  lib/  libexec/  share/  ssl/
And mingw64
D:\prgs\gits\current\mingw64>ls
bin/  doc/  etc/  lib/  libexec/  share/  ssl/
So it looks like it is C:\Program Files\Git\usr\bin.
As explained in "Why is “MINGW64” appearing on my Git bash?", The MINGW64 is the value from the MSYSTEM environment variable.
It is part of MSYS2, which consists of three relatively separate subsystems: msys2 , mingw32 and mingw64.
From "Zsh on Windows via MSYS2" from Borek Bernard:
- msys2 (sometimes called just msys) is an emulation layer — fully POSIX compatible but slow.
- mingw subsystems provide native Windows binaries, with Linux calls rewritten at compile time to their Windows equivalents.
 For example, Git for Windows is a mingw64 binary (unlike msys Git which utilizes the compatibility layer and is therefore slow).
See also "How are msys, msys2, and msysgit related to each other?".