How do I override my header from the theme I chose on WordPress

Question

Everything looks perfect from my Elementor editor when I was creating the website, but after checking it out with the preview link outside WordPress it is was showing something else entirely i.e.

1. The header was displaying the theme's header design not the one created with Elementor header and footer
2. The content of the website seems to be in a box when previewed outside WordPress (check for the pictures)

The picture here is from the editor and this is how it should look like

But this is what I am getting instead