How to archive site page/site ?

You’re not going crazy 😄 modern Site Pages libraries don’t expose a “Download” button the same way document libraries do. Here are a few quick workarounds:

-Use Classic View-

   -In the Site Pages library, click "All pages - View in classic SharePoint"
   -Select your page, then click the ellipsis (…) → "Download a Copy"

-Enable Offline Client Availability-

   -Library settings - Advanced settings - set Offline Client Availability to Yes
   -Sync the library with OneDrive; you’ll then see the files locally and can copy/download them.

-Direct URL trick-

   -Browse to the page’s raw URL:

     
     https://yourtenant.sharepoint.com/sites/YourSite/SitePages/PageName.aspx?web=0
     
   -Right-click the content area and "Save As" to get the .aspx file.

-PnP PowerShell-

   
   Connect-PnPOnline -Url https://yourtenant.sharepoint.com/sites/YourSite -Interactive
   Get-PnPFile -Url "/sites/YourSite/SitePages/PageName.aspx" -Path "C:\Backup" -AsFile