I have had a hard time squaring how contagious it is by external evidence, with how non contagious it is within family units. I finally started seeing families

I have had a hard time squaring how contagious it is by external evidence, with how non contagious it is within family units. I finally started seeing families where it would hit everyone in the family, but there are tons of examples of it not spreading to everyone. Which is practically unheard of for this style of virus. This is particularly pronounced when it comes to kid sicknesses. For the most part, you can just ask if other kids are getting sick to know if your is about to. Covid? We literally carpooled with families and did not catch it from them. Just, how?