I know we’re limited on checkout and can not modify what’s already in place, but can we build a checkout extension, or is there a feature where we can, by default, expand the order/products summary on mobile devices
Just in case this is missed. I am replying so that it can come to top 
Hi @Namish_Kapoor! Unfortunately there’s no way to control whether the order summary is expanded or collapsed on mobile through checkout UI extensions - the collapsed state is baked into the checkout experience by design.
The UX guidelines for checkout extensions confirm this is intentional behavior - the order summary collapses on mobile to save screen space, and only auto-expands when there’s a single product variant in the cart.
A feature request has already been logged for this, but no timeline on if or when it might become configurable. The more a feature is requested, the more likely it is to be considered for development by our product teams so thanks for raising this with us!
Thank you for the info @Donal-Shopify I appreciate it 