Every store, regardless of plan, runs the same Optimized One-Page Checkout, a single-page flow combining shipping, billing, and payment into one screen rather than BigCommerce's older multi-step layout. BigCommerce hosts and maintains that checkout directly, which means consistent performance and PCI compliance, but also means there's no theme-level checkout template to edit the way there is on the rest of the storefront.
Customization at this layer happens through Checkout.js, BigCommerce's SDK for injecting custom scripts and styling into the hosted checkout page, letting a business adjust branding or light behavior without replacing the checkout entirely. It's meant for cosmetic and minor functional tweaks, not for fundamentally restructuring what the checkout captures.