Arduino Uno R3 communicate with ESP8266 (NodeMcu V3)

Hello, I am trying to do a simple data transfer between the Arduino and ESP. I have connected the Rx and Tx to Tx and Rx of each device. Also, I followed the video here Arduino to ESP8266 Data With Serial Communication Using Arduino IDE (Mac OSX and Windows) - YouTube. It's kinda working after I had to add a potentiometer from Arduino Tx to decrease the voltage to 1.9 (initially it was 5) so ESP can read the response. Even though there is no great success, my question is why this is working with 1.9V and not with 1.8V or 2V? Does anyone ever have the same issue before?

The ESP32 is using 3.3V and the Arduino is using 5V. This means, you must lower the voltage of the arduino's tx pin to 3.3V. You also need to up the voltage of the esp32 to the arduino, because it needs at least 4.5V to detect a logical high on an input.

Ok, my problem is that if I set Arduino Tx to 3.3 in the Serial Monitor I get symbols like this ??. If I decrease it more (1.9V) I get the message.
Screenshot -> Screenshot by Lightshot
I send the {"type":"request"} to Arduino and on the screenshot above is the response on ESP

You can't set the voltage in the serial monitor. You need a level shifter ic or at least a voltage divider consisting of resistors for this to work.

Connecting the nodeMCU Tx to the Arduino's RX is fine !

You also need to up the voltage of the esp32 to the arduino, because it needs at least 4.5V to detect a logical high on an input.

No it considers anything over 2.1v as logic HIGH.
The RX pin on the nodeMCU seems to be having some kind of internal pull-up that while in INPUT mode ends up at about 2.4v, with as a result that i had trouble receiving anything the moment i put any resistor in line. Until i put a diode (1n4148) in between the Uno TX to nodeMCU RX with the kathode towards the Uno. When the Uno goes LOW so does the nodeMCU (and current was about 4mA) and when the Uno goes HIGH there is no current, and the nodeMCU is not being pulled LOW. I know there are people that just connect the Uno and the nodeMCU's RX-TX but don't, though i think there is actually a protective circuit which includes a diode on the RX-pin of the nodeMCU, but an extra diode won't hurt. The phenomena with the trimpot is weird, and it may be dependent on the baud-rate as well.