How do I get information about a page, such as featured image, except, and title?

When you save a selected page in the Customiser you’re just saving the post ID of the page, which means you can just pass that value to any function that accepts a post ID as a parameter:

$mytheme_f_page[1] = get_theme_mod( 'mytheme_featured_page_1' );

echo get_the_title( $mytheme_f_page[1] );
echo get_the_excerpt( $mytheme_f_page[1] );
echo get_the_post_thumbnail( $mytheme_f_page[1] );

The first time you use any one of these the full post (page) is cached internally, so you don’t need to worry about getting them one at a time or all at once, or anything like that.