The old checkout let you edit a Liquid template and do almost anything.
Checkout extensibility replaced checkout.liquid with defined extension points — UI extensions, Functions, and branding through the admin.
Checkout Components reached general availability for Plus merchants in 2026, allowing the post-cart flow to be composed with drag-and-drop blocks.
The constraint is real: some customisations that worked in checkout.liquid have no equivalent, and the honest answer is sometimes to drop them.
What you gain is a checkout that stays fast, upgrades itself, and cannot be broken by a bad template edit.
Checkout extensibility, which is a set of defined mechanisms rather than one file.
The full guide has the formats, the deadlines and the worked examples.