I want to ask what is the little numbers on top/over any random number? For example, 75 in² and 125 ft3. What is it called and what does it mean is my question. I couldn't find anything online since I don't know what it is called.
In terms of formatting, numbers and letters that appear in the top half of a line are in superscript. Similarly, numbers in the bottom half of a line would be called subscript. "Super" and "sub" describe their position over or under the main text.
In your examples, these superscript numbers stand for a mathematical exponent or power as applied to a unit of measurement. If I were reading it out loud, I might say "seventy-five inches squared," or (more clearly) "seventy-five square inches." It means that I am talking about a unit of distance extending in two dimensions. It's a measurement of surface area, just like houses can be measured in square feet (ft2) and land can be measured in square miles (mi2). The cubic version (in3) would refer to a measurement across three dimensions, or a measurement of volume.
Other measurements can also be squared or cubed, especially when doing calculations in STEM fields, but square inches/feet/miles are the most common examples of this in the US.
The general term for a number (or other text) written like this is superscript.
The term in2 refers to square inches, which is the amount of area in a square with sides an inch in length.
Likewise, if it were in3, it would be referring to cubic inches, which is the amount of volume in a cube with side lengths of an inch.
3.6 Derived Units— Derived units are formed by combining base units according to the algebraic relations linking the corresponding quantities. Symbols for derived units are obtained by means of mathematical signs for multiplication, division, and the use of exponents. For example, the SI unit for speed is the meter per second (m/s or m·s–1 and that for density is kilogram per cubic meter kg/m3 or kg·m–3). Most derived units have only their composite names, such as meter per second for speed or velocity. Others have special names, such as newton (N), joule (J), watt (W), and pascal (Pa), given to SI units of force, energy, power, and pressure (or stress), respectively.
SAE Technical Standards Board: Rules for SAE Use of SI (Metric) Units, Rev May 1999. https://www.sae.org/standardsdev/tsb/tsb003.pdf