If you want to show the order summary section open on mobile as a default option then you need to use custom CSS.
First, here's how the default view will look: Step 1: Click on the ‘Edit’ button for the selected Checkout template.
You'll see the 'Customizer' opens up.
Step 2: Click on the "Custom CSS" tab
Step 3: Add the CSS given below to show the "Order Summary" on mobile by default.
Step 4: Click on the "Publish" button
This will save your changes.