AAQ
You are correct the 13 colonies who formed the USA agreed to the terms of the US Constitution which included the mechanism (amendment) to change it.
In almost two and one half centuries there have been very few changes regarding voting, but they were major.
The post civil war amendment giving blacks the vote
The early 20th century amendment giving women the vote
The 20th century amendment allowing direct elect of Senators (previously chosen by state legislatures)
The Viet Nam era amendment giving 18 year olds the vote. I protested and fought for this: “Old enough to fight, old enough to vote!”
As for a national healthcare system, the preamble of the Constitution which I had to learn and memorize in 7th grade begins’In order to form a more perfect Union’ this replaces the Articles of Confederation, and states the purposes of providing for the common defense and promoting the general welfare….
Thus a national health plan, or food stamps, WIC are all in the Federal Government’s purview and powers not reserved to the states.
This argument/explanation would be based o. The concept of original intent.
Sorry if it sounds like a lecture, but I still teach 2 law school courses each year.