Hello @beautifullycalm,
To set up the testimonial simply go to Admin Panel > Appearance > Customize > Widget > Front Page Area and place the widget Corporate: Testimonials.
This section is also set up by choosing the category so create the posts as require and then assign it to suitable category and chose that category to display in this section.
PS: Create the category from Admin Panel > Posts > categories.
Hope this helps.
Best Regards!!