Cmd-Shift-. (dot)
is a toggle that will show hidden files and folders until you shut the Finder window. To make them persist, open a Terminal and type
defaults write com.apple.Finder AppleShowAllFiles true
killall Finder
To make the ~/Library/ folder easy to see, use the Cmd-Shift-. (dot)
method and then drag that unhidden folder into the Finder sidebar. It will stay.
Source: www.macworld.co.uk/how-to/mac-software/show-hidden-files-mac-3520878/