I'm using spfx extensions to customize header and footer, but not customize body content or main content of page in sharepoint online.
The upper part of you box, the top navigation and the gray bar below can not be updated with SPFx. You could possible use css to hide elements but that is not a proper solution and could break in the future. You can use CSOM/JSOM/Rest to add or remove links in the top navigation. You should use SPFX webparts to add content below the gray bar.