The correct answer to this question is the Review Ribbon.
In order to insert a comment you can insert one from the Review ribbon at the top. A faster way / shortcut to also insert a comment is to right click on the cell where you want to insert the comment. You can choose to insert a comment and then write the comment using this method, rather than trying to find the icon on the ribbon at the top.
It would probably be the header.
Those aren't binary numbers. In binary, the only digits are 0 and 1.
Cache are the temporary files that are downloaded onto your computer while going on a website, to clear this, you are getting rid of any of the cache. Not what you downloaded yourself, what the internet page downloaded for you.