I just have a question that I had installes the MRtrix3 using the MSYS2, but I can’t find(like the figure) where it is.
It looks like it installed just fine. Are you asking where the installed commands are? You can query that using
pacman if needed:
pacman -Ql mingw-w64-x86_64-mrtrix3
That will list all the files included in the package and where they were stored on your system. But these should all be in your PATH already, you should be able to start using them straight away. E.g. if you type
mrview, hopefully the viewer will open up.