Pretty much all of it is.
But much (but not all) other adopted legislation in the US consists largely (but also often not entirely) in its raw form of English-language patch instructions for codified law like the US Code. So the legislation itself is added to the cumulative register, but then the main effect most people are aware of is a change to codified law. (Actually, that's true of Constitutional amendments to, but the diff is invariably in practice “the following article is proposed as an amendment to the Constitution of the United States, which shall be valid to all intents and purposes as part of the Constitution when ratified ...”; there's nothing stopping the diff from being more in edit form if desired.
But even though most of the text of most legislation is that way, there is lots of uncodified law, too.
(My understanding is that the UK still mostly does uncodified lawn though it does sometimes include specific amendments to earlier named acts in later ones.)
Honestly, though, on a document the size of the US Constitution, there's little impact (and given the original open-ended model, though newer amendments tend to be proposed with expiration dates for ratification that limit the potential problem, the diffing instruction approach would be problematic since the target of the alteration might be reorganized between proposal and ratification.
> I think the decision in the US constitution to replace the Westminster parliamentary system with a presidential system was a step backward.
The US Constitution didn't replace a Westminster-style parliamentary system with a presidential system; the system the US had under the Articles of Confederation wasn't a Westminster-style parliamentary system.
My point is that how the US Code and how the US Constitution are amended is fundamentally different: amendments passed to the US Code say things like "insert this section here", "delete this section", "replace this one with this one". And then the edits are applied, and the US Code is published with that edits applied. The amendments of the US Constitution aren't even made in the form of textual edits.
> My understanding is that the UK still mostly does uncodified law
Even though UK law largely isn't codified, they still mainly print Acts in consolidated form – repealed sections are omitted, amendments are incorporated into the text, etc. The main thing that stops it being a code is you have lots of acts on different topics listed alphabetically, whereas codification would imply merging all those into one big act (or a few big acts) with its contents being organised topically
> The US Constitution didn't replace a Westminster-style parliamentary system with a presidential system; the system the US had under the Articles of Confederation wasn't a Westminster-style parliamentary system.
Actually in a lot of ways the Articles of Confederation was closer to a Westminster style parliamentary system than the US Constitution is. The defining feature of a parliamentary system is the executive is practically subordinated to the legislature, rather than being an independent seat of political power. The Articles of Confederation had that – the national executive was quite limited in extent (there was a treasury, the army, navy, foreign affairs, and the postmaster-general) but it was wholly subordinate to Congress and had no independent seat of power.