If you want to custom header and footer of your theme, first Install this plugin
☞ https://wordpress.org/plugins/header-footer-elementor/
Step 1 – Visit Appearance > Elementor Header & Footer Builder and click on the ‘Add New’.
Step 2 – Select ‘Type of Template’ (header/footer/block). Set its display location and user roles.
Step 3 – Publish it.
Step 4 – Edit it with Elementor and design a section.
Step 5 – Save it and you are done!
You can refer to step-by-step article that will help you set Elementor headers and footers