Explanation
In Excel, the HYPERLINK function can be used to create links. The basic syntax is:
=HYPERLINK("link","link text")
The link itself is a text string that represents a valid link. The link text (called "friendly name" in Excel) is the text displayed to a user.
Ultimately, the goal for the formula in G5 is to build a string like this:
mailto:aya@aa.com?cc=bb@bb.com&subject=subject&body=body
Because the mailto link use several pieces of information, it must be assembled with concatenation.
The formula is bit tricky. While the ampersand is the operator for concatenation in Excel, it is also used to delimit the mailto link parameters (cc, bcc, subject, etc.). This means that some ampersands (&) are used to join text in the formula, and some are embedded in the final result. In the code below, the ampersands in yellow are used for concatenation in Excel. The white ampersands are embedded in the final result:
=HYPERLINK("mailto:"&C5&"?"
&"cc="&D5
&"&subject="&E5
&"&body="&F5,
"link")
Empty mailto parameters
For the sake of simplicity, the formula above does not try to exclude empty parameters from the final result. In quick testing with Gmail and Outlook, missing parameters seem to be handled ignored gracefully. The behavior of other email applications may vary.