First please install and activate Elementor and Architecturer Theme Elements for Elementor plugins before you proceed.
Once you get required plugins installed. Please follow below steps.
Create a page or edit an existing one.
Click “Edit with Elementor” button to open Elementor Page Builder.
Click “Add Template” button and scroll through “Pages” section.
You can filter only Architecturer templates and select demo page you want to import and click “Insert”.
If you have experience Architecturer template issue with Elementor library. Please make sure you update the plugin “Architecturer Theme Elements for Elementor” to v3.4