MacOS doesn’t include an option in Finder to preview Markdown files (.md). There are several solutions available, and one of the quickest and simplest is to install qlmarkdown
When faced with the question “how can I get Quick Look to show previews of Markdown (.md) files?”, the answer is that there’s a project called qlmarkdown that can be installed with Homebrew and solves the problem.
brew install --cask qlmarkdown
From that point on, the association between .md files and the viewer is established
➜ ~ qlmanage -m | grep "md"
com.unknown.md -> /Users/luis/Library/QuickLook/QLMarkdown.qlgenerator (1.3.5)
➜ ~
Note: In my case I had to restart the system for this to work; to allow me to validate its execution.