The Loop Builder enables you to create and design the main item for repeating content (i.e. loops) such as listings, as well as customize its layout on the page. By creating a main item template yourself, you get a lot more flexibility compared to the Posts widget to design your listings and include additional dynamic content. Read these articles for all details:
https://elementor.com/blog/introducing-the-loop-builder/
https://elementor.com/help/how-do-i-build-a-loop-grid/
How to edit the main item template of the Testimonials loop?
Edit the page with Elementor:
To edit the template settings and layout, click "Edit template" or you can directly edit the Testimonials loop template here:
All of the main item's elements are dynamic tags (https://developers.elementor.com/docs/dynamic-tags/), so they pull the individual content of each post/page:
How to edit the Testimonials loop template's query?
Edit the page with Elementor > Open the Query tab and edit the source, order, etc.
How to edit the Testimonials loop slides' content?
You can directly edit the Testimonials loop slides' pages:
Or, go to Dashboard > Pages > locate each page and edit it with Elementor:
Open the page settings and edit the "Title", "Excerpt" and "Featured image":
