LearnDash 3.0 - Focus Mode - Course Navigation - Customize the expand/collapse arrow links

learndash-3-focus-mode-navigation-expand-arrows.css

/**
 * This only works when using the LearnDash 3.0 template.
 * 
 * Use the following CSS to apply a specific background color to the circle
 * around the arrow, and then change the color of the arrow itself.
 *
 * These styles should be copied & pasted to a child theme or the "Additional CSS"
 * area of the WordPress Customizer.
 *
 * Use your own color values for both background-color and color.
 */

.learndash-wrapper .ld-course-navigation .ld-lesson-item-preview .ld-expand-button .ld-icon {
	background-color: #000;
	color: #fff;
	opacity: 1;
}

If you're using LearnDash's video progression feature, and auto-completing the lesson after the video completes, it should change to a checkmark.

But if not, and you're embedding the video directly in WordPress, then the user will need to click the "mark complete" button for the checkmark to change.

https://wordpress.org/support/article/video-shortcode/

1. I don't think that's a valid shortcode.
2. It looks like you've customized the LearnDash focus mode. I also can't see if there are buttons below your video. There should be.
3. I am an expert but I'm not available for hire.

Good luck with your project.

Sorry, I'm all out of help. Good luck.