Where to insert HTML for loader?

I’ve got a child theme.
I want to have a loader like this https://www.w3schools.com/howto/howto_css_loader.asp

But i don’t know where to insert the html?
So far my tests have been unsuccessful on:

– index
– header
– header-home
– templates/homepage-onetone

This one is a hard one to nail! I don’t want to use a plugin!
Can anyone shed some light?