Strcmp or strcat , can we compare or concatenate with offset ? how please?

Try this

void setup()
{
    Serial.begin(115200);
    char Buff1[30] = "abcdefghijklmnopqrst";
    char Buff2[30] = "123456abcdefghijklmnopqrst";

    if (strcmp(Buff1, Buff2 + 6) == 0)
    {
        Serial.println("COMPARE TRUE");
    }
    else
    {
        Serial.println("COMPARE FALSE");
    }
}

void loop()
{
}