Any tool installed via asdf is available on any directory as long as you are accessing that directory via a shell spawned with a .profilerc or similar which contains your asdf configuration.

> asdf Node installs aren't like Python virtual environments

Correct, neither should they be.

> they're centrally installed and one Node version (and its packages) is shared across all diretories

sure, they are, and that's by design. You're conflating a runtime manager with a package manager. Venvs are _not_ runtime manager, the moment you need another Python version you're done for. asdf.vm is _not_ a package manager, the moment you want package isolation while working on an asdf install is the moment you install yourself pipenv, poetry, pdm or use python venvs for that.

> that want it + global tools.

which is achieved as I've shown below. Still, there was no reason in your usecase to modify or play with globally installed tools besides asdf, through which you can then define global runtime versions and those global versions will hold your global tools, usable wherever.