Usually, troubleshooting involves accessing the user Library folder. There is a top level Library folder and a top level System/Library folder. Both are visible. But both are folders that you really don't need to access most of the time. Unfortunately, the user Library folder is only sort-of visible.
Using the alt key in the Go menu
If you press the alt key then the Go menu has an entry for the user Library:
But we want to unhide the Library permanently.
Add the Library folder to the sidebar
There is a totally useful key combo to make invisible files visible. We are now going to use this with the user Library folder.
Navigate to your user folder:
Press "Cmd-shift-." Scroll a little bit down until you see the Library folder. Here comes the trick. Drag the Library folder to the sidebar:
You can now always access the Library folder:
Don't forget to press "Cmd-shift-." again to make the invisible files invisible again.
Using the Terminal
When I have screwed something up and need to access the Library folder in my backup then I needed to use the "Cmd-shift-." key combo I mentioned above to see the Library in the backup. We are now going to fix this.
Open Terminal with Spotlight or Alfred:
Type or copy into the Terminal window:
chflags nohidden ~/Library/
Press enter to execute the command:
And after executing the command I see the Library folder of my backup.