I think that you're using JA Builder template + T4 Blank template + T4 Page Builder, those are different products with different setup so it may cause the confusion.
If you want to use T4 Blank template + T4 Page Builder, you will have to configure the layout of template a bit to achieve that.
- Build a landing page with T4 Builder:
As you know T4 Builder is kind of Joomla component, its content: blocks, addons, ... you edited via T4 Builder will display in the 'component' - main content block of a page.
So in order to have a landing page, you need to configure the Joomla template - in this case is T4 Blank template - to have 'component' block only. I just setup a sample for you with this template style: t4_bs5_blank - landing and assigned this template style to the menu item page of T4 Builder: Landing 2022 and you can see the homepage is loading that T4 Builder page now
- Like I mentioned in point 1, T4 Builder is different from JA Builder.
In JA Builder, you can build a header block and use it for entire pages on your site with JA Builder template.
While with T4 Builder, it doesn't have that option, all of its blocks, elements will show up in 'component' block of Joomla template you use (T4 Blank template).
So with any landing page , T4 Builder page, you can use the template style I created for you. For normal pages, like login page of your site ingresar you should build a normal layout for T4 Blank template: t4_bs5_blank - default - I created for you. And add modules/ sections as you wish: header / footer / ... In your video, it's assigned to layout of landing page so only login form shows up without header + footer ...
If you're new to T4 framework, you can check out this documentation for more info: https://www.joomlart.com/documentation/t4-framework/layout-builder
I hope this makes senses.