Finding Open/closed loop transfer function
Start by putting some equations on it: -







simulate this circuit &ndash; Schematic created using CircuitLab

The output of A is called Vout, but clearly ....

\$V_{out} = A(V_{in} - B\cdot V_{out})\$

This boils down to \$\dfrac{V_{out}}{V_{in}} = \dfrac{A}{1+AB}
\$