Couldn't understand an example used in explaining fundamental and derived quantities In my book under topic of fundamental and derived quantities, there is an example which is supposed to explain this concept and it goes like this,

As a simple example, if a unit of length is defined, a unit of area is automatically obtained. If we make a square with its length equal to its breadth equal to the unit length, its area can be called the unit area. All areas can then be compared to this standard unit of area.

What I don't understand is that where is the numerical value that should be associated with the unit of length while saying about length and how can the 'area' of this square be a unit of area and also how it became standard as the example says in the last sentence and what does 'All areas can be compared to this standard unit of area'(the last sentence) supposed to mean. I am having a hard time understanding this example. I need assistance
 A: The metre, or the kilometre, or the mile, or any other unit of length is a product of humans (the Planck Units are an exception, though). You are free to “create” a new unit to measure length. For example, you may call the straight-line distance between the tip of your middle finger and the extreme-most point of your shoulder when your arm is extended “$1$ arm’s length”. This newly defined unit of length is no less valid than any other. Now, recognise that the unit of area is of the format: (unit of length)$^2$. Area can always be expressed as the product of two lengths. This is why if we define a unit of length, a unit of area is automatically obtained. Now, imagine a square with side length equal to $1$ arm’s length. The area of the square is $1$ (arm’s length)$^2$. This is a result of the fact that the area of a square of side length $s$ is equal to $s^2$, and $1^2=1$. The “numerical value that should be associated with the length” should be understood to be $1$ whenever we talk about a “unit length”. Similarly, the numerical value associated with a “unit area” is 1. The numerical value associated with a “unit ~quantity~” is 1.
