how to open vim in a specific directory

Without any configurations to Nerdtree, vim /path/to/file should open up Nerdtree on file if it is a directory (hijacking Netrw, vim's default file explorer, which would have opened on the directory otherwise). If you're sitting in Project/backend/, you should be able to run vim . to open the backend/ directory.

If you're asking how to open Nerdtree rooted at that directory from within vim, you can run :NERDTree /path/to/dir, and you'll have opened a Nerdtree rooted at dir, which you may now toggle with :NERDTreeToggle. Note that when you open a directory with vim /path/to/dir from the commandline, you'll have opened the directory file with Nerdtree however the vim window's Nerdtree won't actually be rooted anywhere so using :NERDTreeToggle (or :NERDTree) will use vim's current working directory, which will by default be the folder you were in when you ran vim, and not necessarily /path/to/dir.

It might be worthwhile writing a little script in your config to store the initial directory you opened vim in in a variable so that you can have NERDTree original_dir bound to a key if you want to be able to easily open Nerdtree on the original directory you opened. Perhaps someone who knows more about Nerdtree might know of an easier way.