What is the explicit map of the open embedding $B^- \to G/U$?

Question

Let $G=GL_n$ and $B^-$ the set of lower triangular matrices in $G$. It is said that there is an open embedding $B^- \to G/U$. What is the explicit map of $B^- \to G/U$. For example, in the case of $GL_2$. We have every element in $B^-$ is of the form $\left( \begin{matrix} a & 0 \\ c & d \end{matrix} \right)$. What is the images of elements in $B^- \to G/U$. Thank you very much.

Answer 1

Assuming $U$ is the set of upper-triangular unipotent matrices. Think about $B^-\to G\to G/U$.