Real communication between arduino and NodeMCU

Why not? People do this all the time. Start on this forum with Serial Input Basics.

It is pretty simple to design your own command/communications protocol. For example, serial commands could look something like this:

<s4,1> set digital output pin 4 to 1
<p9,150> set PWM on pin 9 to 150
<g2,0> get analog input on A2