Source Transformation (complex circuit)
Maybe a simple statement about voltage and current sources in parallel or series might help: -

A voltage source in parallel with a resistor appears to be a pure voltage source for any other connected components.
A current source in series with a resistor appears to be a pure current source for any other connected components.
A voltage source in parallel with a current source - the voltage source wins hence it defines the terminal voltage.
A voltage source in series with a current source - the current source wins hence it defines the terminal current.