The starting point is always the uniform distribution $$\begin{equation*} p(x)dx=\left\{\begin{array}{cc} dx & 0 \le x \le 1\\ 0 & else\end{array}\right. \end{equation*}$$ with $p(x)=1$ and satisfying $$\begin{equation*} \int_{-\infty}^{\infty}p(x)dx=1. \end{equation*}$$ All random number generators use the uniform distribution to generate numbers $x\in [0,1]$.