Hi,
I was not aware of the whole process as it was carried out by the company, not me, but it is my understanding that they receive a great number of resumes after advertising.
I guess it depends on the job you are being considered to, if it is a really specific job or not.
In a specific job, I guess an advertise may draw something like 30 resumes? I do not have the exact number.
I also do not believe they interview all of them.
A filter may be applied at this point, as they might want to stick to the requirements of the position.
I am not sure I can answer all your questions, but I might try.
Maybe some expert here can fill in the blanks we leave.