I don’t have a Mac, so I can’t test this, but I think all you have to do is download the tar file and then extract it:
$ curl -O https://julialang-s3.julialang.org/bin/mac/x64/1.8/julia-1.8.2-mac64.tar.gz
$ tar xzf julia-1.8.2-mac64.tar.gz
$ ./julia-1.8.2/bin/julia # <-- starts Julia
You would probably also want to update your PATH so you can start Julia with julia from any directory.