No package manager on brand new macOS Catalina
The only package manager that’s somewhat default is /usr/bin/easy_install for python. You have to choose and install any of the leading open source type package managers. 

For brew, this has been the command to download and run code from the internet. 

/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"


If this thought makes you cringe, alternate install paths are safer than trusting dns and scripts without having a chance to inspect them. 

mkdir homebrew &amp;&amp; curl -L https://github.com/Homebrew/brew/tarball/master | tar xz --strip 1 -C homebrew