Code to "Show Posts" and "Do a query on the posts database and filter

Hello,

I purchased a “Testimonials for WordPress” plugin to manage and show testimonials on my site. It uses custom post type for my testimonials. It only included a shortcode to show one testimonial on the page. I want to show all testimonials. The developer instructed me to

“use the same code to show posts in your website. Do a query on the posts database and filter the result to show only the records where post_type=testimonial”

I don’t really know what that means or how to do it. Can anyone provide me with the code and instructions to do that? Can you please provide the code and tell me which files in my theme to add it to, and where to add it?

I appreciate any help anyone can provide!