How to inject HTML like div for footer in OOB master page in SPO to apply custom Branding, if not to use custom Master page

Hi All,

We are planning to use OOB master page for custom Branding by injecting custom CSS and JS in SharePoint Online instead of creating new custom Master page, so Just want to know, how to inject our custom html like our custom div for footer, Navigation control for iPad/iPhone in out of box master page.

We have gone through this link to make site responsive using custom action injection https://github.com/OfficeDev/PnP-Tools/tree/master/Solutions/SharePoint.UI.Responsive