By default, use-package assumes that you are using the Emacs built-in package.el package manager. We expect that most users will find that it is capable enough, even for advanced use cases.
However, some users might prefer to use a third-party package manager
for a specific circumstance or use case. By setting the user option
use-package-ensure-function to the name of a function, you can
direct :ensure to use a different package manager for
installing packages.
For more details, please see the documentation of the package manager you are using. If you run into any bugs, it is often best to report them directly to the developers of that package manager.