When you click on a skill icon, instead of selecting "Start" (or "Practice" if you've reached level 5) there will usually be an icon that looks like a lightbulb. That will take you to the tips and notes for that skill. They are not available on the app, so this will only work if you're using a browser (Chrome, Safari, etc.)
I never thought of typing the url manually, but that should work, too :-)
No, they are not always the same.
This is a web page that has helped me a lot with this. It starts off sounding really confusing, but they found a nice way to simplify things! (The simplified explanation starts at "If you understand 2 important principles …")