How to download rendered content of *.aspx SitePage using SharePoint Online Rest API

Question

We are trying to download rendered content of SitePages ( relativeURL: /SitePages/fileName.aspx) But we are getting non rendered content;

We need help to achieve the same;

Also, When we try to download .aspx from '/_catalogs/masterpage/', we get proper rendered content;

What is the difference between these two .aspx files.