Change default excerpt to custom post field

Tis tutorial provides the steps to change default excerpt to show custom post field. This tutorial uses the flexible content but can be work on any field.

In child-theme/functions.php, add

add_filter('the_excerpt', 'your_function_name');

function your_function_name($excerpt) {
  if( have_rows('flexible') ):
  
    while ( have_rows('flexible') ) : the_row(); 
        if( get_row_layout() == 'flexible_name' ): 
      
         $my_acf_field = wp_trim_words(get_sub_field('custom_excerpt'), 100);

         echo ''.$my_acf_field.'';
        endif;
    endwhile;
    else:
    endif;
    
}

More: https://www.advancedcustomfields.com/resources/flexible-content/