Serial communication arduino mega and D1 Wemos Mini
Apart from ensuring you're wiring TX -> RX and connecting all grounds, you can use SerialTransfer.h to automatically packetize and parse your data for inter-Arduino communication without the headace. The library is installable through the Arduino IDE and includes many examples.

Here are the library's features:


  This library:
  
  
  can be downloaded via the Arduino IDE's Libraries Manager (search "SerialTransfer.h")
  works with "software-serial" libraries
  is non blocking
  uses packet delimiters
  uses consistent overhead byte stuffing
  uses CRC-8 (Polynomial 0x9B with lookup table)
  allows the use of dynamically sized packets (packets can have payload lengths anywhere from 1 to 254 bytes)
  can transfer bytes, ints, floats, and even structs!!
  


Example TX Arduino Sketch:

#include "SerialTransfer.h"

SerialTransfer myTransfer;

void setup()
{
  Serial.begin(115200);
  Serial1.begin(115200);
  myTransfer.begin(Serial1);
}

void loop()
{
  char buff[] = "hi";

  myTransfer.txObj(buff, sizeof(buff));
  myTransfer.sendData(sizeof(buff));
  delay(100);
}


Example RX Arduino Sketch:

#include "SerialTransfer.h"

SerialTransfer myTransfer;

void setup()
{
  Serial.begin(115200);
  Serial1.begin(115200);
  myTransfer.begin(Serial1);
}

void loop()
{
  if(myTransfer.available())
  {
    char buff[40];
    
    myTransfer.rxObj(buff, sizeof(buff));
    
    Serial.println("New Data: ");
    Serial.write(buff, sizeof(buff));
    Serial.println();
  }
  else if(myTransfer.status &lt; 0)
  {
    Serial.print("ERROR: ");

    if(myTransfer.status == -1)
      Serial.println(F("CRC_ERROR"));
    else if(myTransfer.status == -2)
      Serial.println(F("PAYLOAD_ERROR"));
    else if(myTransfer.status == -3)
      Serial.println(F("STOP_BYTE_ERROR"));
  }
}