Forked twentytwenty template-parts/pagination.php to support amp-next-page: https://wordpress.org/support/topic/infinite-scroll-feature-in-amp/

functions.php

<?php

// Add this code to the theme's functions.php. 
add_action(
	'wp_head',
	static function () {
		if ( mytheme_is_amp() ) {
			echo '<meta name="amp-experiments-opt-in" content="amp-next-page">';
		}
	},
	1
);

function mytheme_is_amp() {
	return function_exists( 'is_amp_endpoint' ) && is_amp_endpoint();
}

pagination.php

<?php
/**
 * A template partial to output pagination for the Twenty Twenty default theme.
 *
 * @link https://developer.wordpress.org/themes/basics/template-files/#template-partials
 *
 * @package WordPress
 * @subpackage Twenty_Twenty
 * @since 1.0.0
 */

/**
 * Translators:
 * This text contains HTML to allow the text to be shorter on small screens.
 * The text inside the span with the class nav-short will be hidden on small screens.
 */

$prev_text = sprintf(
	'%s <span class="nav-prev-text">%s</span>',
	'<span aria-hidden="true">&larr;</span>',
	__( 'Newer <span class="nav-short">Posts</span>', 'twentytwenty' )
);
$next_text = sprintf(
	'<span class="nav-next-text">%s</span> %s',
	__( 'Older <span class="nav-short">Posts</span>', 'twentytwenty' ),
	'<span aria-hidden="true">&rarr;</span>'
);

$posts_pagination = get_the_posts_pagination(
	array(
		'mid_size'  => 1,
		'prev_text' => $prev_text,
		'next_text' => $next_text,
	)
);

// If we're not outputting the previous page link, prepend a placeholder with visibility: hidden to take its place.
if ( strpos( $posts_pagination, 'prev page-numbers' ) === false ) {
	$posts_pagination = str_replace( '<div class="nav-links">', '<div class="nav-links"><span class="prev page-numbers placeholder" aria-hidden="true">' . $prev_text . '</span>', $posts_pagination );
}

// If we're not outputting the next page link, append a placeholder with visibility: hidden to take its place.
if ( strpos( $posts_pagination, 'next page-numbers' ) === false ) {
	$posts_pagination = str_replace( '</div>', '<span class="next page-numbers placeholder" aria-hidden="true">' . $next_text . '</span></div>', $posts_pagination );
}

if ( $posts_pagination ) { ?>

	<?php if ( mytheme_is_amp() && ! is_paged()  ) : ?>
		<?php
		// @todo This is not the ideal way to obtain the subsequent pages, but WordPress doesn't provide such a way.
		// @todo The page titles aren't ideal.
		$pages = array_filter( array_map(
			static function( $link ) {
				if ( preg_match( '#<a.+?href="(.+?)">(.+?)</a>#s', $link, $matches ) ) {
					$amp_url = html_entity_decode( $matches[1] );
					$title   = html_entity_decode( $matches[2] );
					return [
						'title'  => $title,
						'ampUrl' => $amp_url,
						'image'  => get_site_icon_url(),
					];
				}
				return null;
			},
			paginate_links( [ 'type' => 'array' ] )
		) );

		$amp_next_page = [
			'pages'         => array_values( $pages ),
			'hideSelectors' => [
				'#site-header',
				'.header-footer-group',
				'#site-footer',
				'#wpadminbar',
			],
		];
		?>
		<amp-next-page>
			<script type="application/json"><?php echo wp_json_encode( $amp_next_page ); ?></script>
		</amp-next-page>

	<?php else : ?>

		<div class="pagination-wrapper section-inner">

			<hr class="styled-separator pagination-separator is-style-wide" aria-hidden="true" />

			<?php echo $posts_pagination; //phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- already escaped during generation. ?>

		</div><!-- .pagination-wrapper -->
		<?php
	endif;

}