use-package is a macro, NOT a package manager. It can call either package.el (the default emacs package manager which uses MELPA but other package repo can be added) or straight.el (the new package manager alternative which allows to use package repos as package.el BUT also allows to use custom defined, git-sourced package repos)

Leaf in emacs provides a better configuration and package loading in init.el. It is intended as plugin replacement for use-package.