Checkout.liquid customization not showing on theme preview

[emlez]

Customizations on checkout.liquid file don't show up on the preview of a theme I'm editing.
I'm working on a store on the Shopify Plus plan, so checkout.liquid customizations should work.
I don't want to test on the live/prod theme; that's why I'm only interested in it working on a theme preview.

[Jason]

Are you sure that checkout is activated on that shop (and I don't mean just having the file created).
If you are building with that file it's going to make sense to start looking at extensions instead.

https://shopify.dev/docs/api/checkout-ui-extensions