Why are the Undo and Redo arrow icons commonly round?

It indicates that you are returning to a place you've already been, either forwards (redo) or backwards (undo). The refresh button being a near circle indicates that you are doing something and returning right where you started, neither forwards nor backwards.

1. What icon is used to represent the Undo feature?
2. What is the symbol of redo?

What icon is used to represent the Undo feature?

Click the back arrow icon to undo.

To undo a change made to your image, simply click on the backward arrow icon to backtrack.

What is the symbol of redo?

To redo something you've undone, press Ctrl+Y or F4.