Within Google Apps Docs and Sheets, Add-ons are available. You can add whatever you think will be most useful to you. While I haven't experimented with Sheets much yet, Google Docs has several really useful Add-ons. (If you ever worked with Scripts in the Google Apps of two years ago, Add-ons pretty much replaces them, though if you have an existing script in place, it will sometimes still work.)
I took a look at TextHelp Study Skills that allows you to highlight text in Docs. (Click to make pictures bigger.)
You may also insert everything from Venn diagrams to organization charts to floorplans with Gliffy Diagrams.
Anything you create in Docs, Sheets, and Slides or upload to Drive is automatically set to private, which means only you can view it. But when you do want to share a file or folder, there are a few easy ways:
Like always, just click the big, blue “Share” button at the top of an open file, or if you are in your Drive list, you can now click the person icon at the top of the page after you’ve selected one or more files.
A box will pop up, and from there, you can add individual people to share with directly. But as a new addition, if you want to create a unique shareable link, you can now just click the “Get shareable link” button.
The shareable link will be set to view-only by default. And if you are ever wondering whether you’ve already made the link shareable, just look at the icon next to the button. Green means yes, grey means no, just like on your Android device.
If you want anyone with the link to be able to do more with your file, like leave comments and suggestions or make edits, just change “can view” to “can comment” or “can edit” in the dropdown. Check your accounts for these updates over the coming days.