You can’t print an array in that way.

If you want to print an array of hexadecimal value (with two digits), you have to use sprintf function and change declaration of b array.

uint8_t b[]= {0x7E, 0x00, 0x1C, 0x90, 0x00, 0x13, 0XA2, 0x00, 
              0x41, 0x58, 0x1C, 0xCB, 0xFF, 0xFE, 0xC1, 0x7C, 
              0x00, 0x0F, 0x00, 0x01, 0x00, 0x00, 0x37, 0x46, 
              0x46, 0x46, 0x00, 0x00, 0x00, 0xE7};
int i;

void printHex(uint8_t num) {
  char hexCar[2];

  sprintf(hexCar, "%02X", num);
  Serial.print(hexCar);
}

void setup() {
  Serial.begin(9600);
}

void loop() {
  for(i=0; i<sizeof(b); i++){
    printHex(b[i]);
 }

 Serial.println();
 delay(500);
}

The output produced is:

7E001C900013A20041581CCBFFFEC17C000F0001000037464646000000E7 7E001C900013A20041581CCBFFFEC17C000F0001000037464646000000E7 7E001C900013A20041581CCBFFFEC17C000F0001000037464646000000E7

Simple brute force method, is to write a routine as:

void p(char X) {

   if (X < 16) {Serial.print("0");}

   Serial.println(X, HEX);

}

And in the main code:

p(byte1);  // etc.

you can use bitwise operators, in particular Bitshiftleft "<<" and than Bitwiseor "|".

Fron Arduino documentation:

Bitshiftleft "... The left shift operator << causes the bits of the left operand to be shifted left by the number of positions specified by the right operand. ..."

Bitwiseor "... The bitwise OR operator in C++ is the vertical bar symbol, |. Like the & operator, | operates independently each bit in its two surrounding integer expressions, but what it does is different (of course). The bitwise OR of two bits is 1 if either or both of the input bits is 1, otherwise it is 0. ..."

The type of the variable have to be changed from byte to int, but beware that You have to choose between int or unsigned int based on your needs. An int represented as binary has the highest value (most left) used to define if the number is negative or positive.

You can start from something like this:

byte digitLeftZero[8] = {0b0000, 0b0110, 0b1001, 0b1001, 0b1001, 0b0110, 0b0000, 0b0000};
byte digitRightZero[8] = {0b0000, 0b0110, 0b1001, 0b1001, 0b1001, 0b0110, 0b0000, 0b0000};
byte digitDoubleZero[8] ;

byte a = 0 ;
byte b = 0 ;
byte c = 0 ;


void setup() {

  // open the serial port at 9600 bps:
  Serial.begin(9600);

  // Loop through arrays
  for (byte i = 0; i <= 7; i++) {

    a = digitLeftZero[2] ;
    b = digitRightZero[2] ;

    // debug
    Serial.print(i);
    Serial.print(" - ") ;
    Serial.print(a, BIN);  // print as an ASCII-encoded binary
    Serial.print(" - ") ;
    Serial.print(a << 4, BIN) ;
    Serial.print(" - ") ;

    c = a << 4 | b ;

    // output as binary representation
    Serial.println(c, BIN);  // print as an ASCII-encoded binary

    // assign values to target array
    digitDoubleZero[i] = c ;
  }
}

void loop() {

}

There is a tutorial in Arduino website that can be useful. "Bit math with Arduino" https://docs.arduino.cc/learn/programming/bit-math

You can't return stack allocated memory from a function. Once the function call ends that data will be reclaimed when the stack pointer is updated. You need to either dynamically allocate the memory on the heap like so ...

byte* find_fix(){
    byte* payload = new byte[9]; // Allocate memory on the heap

    return payload;
}

void loop() {
    byte* payload = find_fix();
    delete[] payload; // Free memory once it is no longer needed
}

Or if you know the max size of the array, you can pass in memory to the find_fix function like so ...

void find_fix(byte* payload) {
    ....
}

void loop() {
    byte payload[9];
    find_fix(payload);
}