MacOS Finder has a nice feature where you can hit spacebar and it’ll show you a quick preview of the file. It works with images, videos, text files and such but not with Markdown out of the box.
I use sbarex/QLMarkdown extension that renders markdown files into a pretty preview.
It can be installed from Homebrew with
brew install --cask qlmarkdownand might require manual opening once to confirm for the operating system’s security that it’s okay to run.