Add sidebar to static pages?

in your child theme, create a file with the file name sidebar-page.php, containing this code:

<?php
/**
 Template Name: Sidebar Page
 */

get_header(); ?>

<div class="wrap">
	<div id="primary" class="content-area">
		<main id="main" class="site-main" role="main">

			<?php
			while ( have_posts() ) : the_post();

				get_template_part( 'template-parts/page/content', 'page' );

				// If comments are open or we have at least one comment, load up the comment template.
				if ( comments_open() || get_comments_number() ) :
					comments_template();
				endif;

			endwhile; // End of the loop.
			?>

		</main><!-- #main -->
	</div><!-- #primary -->
	<?php get_sidebar(); ?>
</div><!-- .wrap -->

<?php get_footer();

add this to functions.php or your child theme:

add_filter( 'body_class', 'sidebar_page_body_classes', 12 );

function sidebar_page_body_classes( $classes ) {
    if ( is_page_template( 'template-parts/sidebar-page.php' ) && !is_front_page() ) {
		if( in_array('page-two-column', $classes) ) {
        	unset( $classes[array_search('page-two-column', $classes)] );
			$classes[] = 'page-one-column'; 
		}
		$classes[] = 'has-sidebar'; 
    }
	return $classes;
}

https://developer.wordpress.org/themes/advanced-topics/child-themes/
https://developer.wordpress.org/themes/template-files-section/page-template-files/
https://developer.wordpress.org/reference/functions/body_class/