What makes an inch an inch?

There are actually a few different standards that are used, and there are agencies that are responsible for publishing and updating them.

For instance, for the International System of units, there's an agency in Paris called the BIPM (Bureau international des poids et mesures, or "International Bureau of Weights and Measures", that sets the standards for metric units. Initially these were identified based on physical standards- e.g. there used to be a piece of metal somewhere in Paris that was the international reference for "one kilogram". These days they have moved away from that to use more basic physical standards that can be replicated. For instance, distance is based on the distance that light travels in a certain period of time, which itself is based on measures from atomic clocks, and so forth. The idea is that these new definitions are based on fundamental physical constants rather than a physical reference, so any lab should be able to measure them.

In most countries, the government is officially responsible for setting the standards for weights and measures in that country. Most have a national standards agency that serves as the official reference for that country- often they will use some agreed international standard but may modify it based on that country's particular needs, or units that are customarily used in that country.

For instance, US customary units like inches and so forth are actually defined based on the SI system of units. For instance, a yard is defined as 1 yard = 0.9144 meters, and so forth.

The USA is one of only seven countries that doesn't use the SI system as the official system, although it is still widely used in the USA anyway.

This will be hard to believe but the inch was based on the width of King Edgar’s thumb. The foot was determined by the length of King Charlemagne foot and the yard... The distance from King Henry I’s nose to his fingertips.

Found this by looking up “how the ‘foot, yard and inch’ came to be”.

The width of a man's thumb. That's what the inch was based off of. That's why most men can use their thumb to estimate an inch. If you're looking for a universal starting point in the English system of measures, you are going to be looking for a long time because there isn't one, like how many inches in a foot is simply derived from how many man's thumb widths it took to reach a man's foot length. If you ever decide to shift your focus to the metric system, however, you will find more satisfaction.