See https://wordpress.stackexchange.com/q/341855 for details.

HOWTO.md

For the Twenty Nineteen theme (tested on version 1.4):

The homepage by default uses the index.php template, and in that file, the posts are displayed like this:

// Load posts loop.
while ( have_posts() ) {
	the_post();
	get_template_part( 'template-parts/content/content' );
}

And in template-parts/content/content.php, you'd see the following:

the_content(
	sprintf(
		wp_kses(
			/* translators: %s: Name of current post. Only visible to screen readers */
			__( 'Continue reading<span class="screen-reader-text"> "%s"</span>', 'twentynineteen' ),
			array(
				'span' => array(
					'class' => array(),
				),
			)
		),
		get_the_title()
	)
);

And that displays the full post content.

If you want to show just the post excerpt, change the above code to this:

if ( is_singular() ) {
	the_content(
		sprintf(
			wp_kses(
				/* translators: %s: Name of current post. Only visible to screen readers */
				__( 'Continue reading<span class="screen-reader-text"> "%s"</span>', 'twentynineteen' ),
				array(
					'span' => array(
						'class' => array(),
					),
				)
			),
			get_the_title()
		)
	);
} else {
	the_excerpt();
}

That should work, but you might want to create a child theme so that your modification wouldn't be wiped when you update Twenty Nineteen.

Hi, I just checked the directory /wp-content/themes/twentyseventeen/template-part/post/ but don't see a file called content.php. Is there a way to reinstall this theme if the file is missing for whatever reason?

@GreenGablesFan, see the updated HOWTO.md.

And I suggest you to write your own answer for that question.