Anyone know how can I put the box in image in the navigation area without using SPFx just CSS only.
You cannot inject custom CSS in modern pages without using SPFx.
You can use SPFx application customizer to inject the custom CSS in SharePoint online modern pages. Check more information at:
Note: DOM manipulation & CSS customizations are not recommended by Microsoft and some of your customization may break if Microsoft changes HTML element id/classes in new release updates.
As far as I know, custom css are not supported by sharepoint modern page any more. We can only use spfx to custom the modern page. Please refer to following document