Convert serial.read() into a useable string using Arduino?

Unlimited string readed:

String content = "";
char character;
    
while(Serial.available()) {
     character = Serial.read();
     content.concat(character);
}
      
if (content != "") {
     Serial.println(content);
}

From Help with Serial.Read() getting string:

char inData[20]; // Allocate some space for the string
char inChar=-1; // Where to store the character read
byte index = 0; // Index into array; where to store the character

void setup() {
    Serial.begin(9600);
    Serial.write("Power On");
}

char Comp(char* This) {
    while (Serial.available() > 0) // Don't read unless
                                       // there you know there is data
    {
       if(index < 19) // One less than the size of the array
       {
           inChar = Serial.read(); // Read a character
           inData[index] = inChar; // Store it
           index++; // Increment where to write next
           inData[index] = '\0'; // Null terminate the string
       }
    }

    if (strcmp(inData,This)  == 0) {
       for (int i=0;i<19;i++) {
            inData[i]=0;
       }
       index=0;
       return(0);
    }
    else {
       return(1);
    }
}

void loop()
{
    if (Comp("m1 on")==0) {
        Serial.write("Motor 1 -> Online\n");
    }
    if (Comp("m1 off")==0) {
       Serial.write("Motor 1 -> Offline\n");
    }
}

You can use Serial.readString() and Serial.readStringUntil() to parse strings from Serial on the Arduino.

You can also use Serial.parseInt() to read integer values from serial.

int x;
String str;
    
void loop() 
{
     if(Serial.available() > 0)
     {
        str = Serial.readStringUntil('\n');
        x = Serial.parseInt();
     }
}

The value to send over serial would be my string\n5 and the result would be str = "my string" and x = 5