Hi you,

#1. Cart page, Checkout page: using shortcode and installed on existing pages of Woocommerce. To customize these pages:
Method 1: overwrite the template files of folder cart and checkout.

Follow guide at Overriding templates via a theme
Method 2: Use Elementor Pro or Elementor add-ons
How To Create WooCommerce Checkout Page Using Elementor
Customize Your Elementor Cart and Checkout Pages for WooCommerce

#2. Cart sidebar: We use Cart widget Woocommere, so to customize you have to overwrite this file