In Finder there is a Go menu. The Library folder is usually hidden from the user, but if you press the Option (Alt) key it will appear in the list, and you can go into the user’s library. Once you’re in there you’ll see Application Support, etc.
The way you wrote the path is missing a bit. The user’s name would normally be in there. So for me it would be:
/Users/colin/Library/Application Support/SketchUp 2018/WebCache/
Another useful trick to know is that ~/ is a shortcut to the current user’s folder. You can write that same path as:
~/Library/Application Support/SketchUp 2018/WebCache/
and it wouldn’t matter what the user’s name is. There is also a contextual menu that lets you select the text of any path, and have that location opened up for you in Finder. Like this: