How does the plugin XlsxImportExport work, and what can do with it, and how do I use it? I am asking because I have an excel sheet that I want to convert to SMath, and I would like to see if this function will help.

You're best bet is to go straight to the topic for this extension. https://en.smath.com/forum/yaf_postst1860_Xlsx-Import-Export-plugin.aspx

There is a documentation archive that may answer some of your questions.