Content Restriction but allow public REST API

You can use the REST_REQUEST constant to detect if this is a REST request, and if it’s not, apply your filter(s).

Example:

add_filter( 'the_content', 'wpse418531_use_excerpt' );
/**
 * Replaces the content with the excerpt, unless this is a REST request.
 *
 * @param  string $content The post content.
 * @return string          The filtered post content.
 */
function wpse418531_use_excerpt( $content ) {
    if ( is_admin() ) {
        // If this is an admin page, returns the content unchanged.
        return $content;
    }
    if ( defined( REST_REQUEST ) && true === REST_REQUEST ) {
        // If this is a REST request, returns the content unchanged.
        return $content;
    }
    $content = get_the_excerpt();

    // If you want to add a note about your app, you can do it here.
    // If not, delete these lines.
    $content .= '<p>See more on <a href="https://example.com/download/my/app/">my app</a>.</p>';

    return $content;
}

Note: This code is untested. Don’t use it on a production site till you’ve ensured it works properly. It’s also meant as a starting point for you, not a finished product.

References

• rest_api_loaded() which contains REST_REQUEST (thanks to this answer for the info)
• the_content
• get_the_excerpt()