Someone explain this probability

This question is from jee mains 2022. Everywhere I see the solution, people have only found out the numbers which are divisible by 3 and have considered that to be the final answer without considering if they are divisible by 7. Can someone explain how is that ? How do you know that every 6 digit number formed by using 1 and 8 only will be a multiple of 7 If it's a multiple of 3 ??