why does multiplying a fraction e.g. enumerator/divisor with divisor/enumerator give 1? I just found out that if you want to get 1 with the fraction: $$\frac{5}{2}$$ Then you multiply it with: $$ \frac{2}{5} $$ Does anyone have a good way to think about this?

This is called The Inverse Property of Multiplication. Take $x$, then

$$x \cdot \frac 1 x = 1$$

In your case of $x=\frac 5 2$,

$$ \frac 5 2 \cdot \frac{1}{\frac 5 2} = \frac 5 2 \cdot \frac 2 5 = 1 $$