Even if a mastectomy it still can be confined to breast, many wonen choosecto or need a mastectomy as opposed to lumpectomy just due to size and ir choice. It is standard practise to take a sentinal node biopsy (that is taking 1 to 3 nodes closest to the tumour) to check if remain cancer free. If they suspect beforehand or know beforehand then they do an auxiliary node clearance straight up to start with. Even if in nodes , it doesn't mean it's not curable.
I've included a link which has information for you about breast cancer. Also included in that is a link to order your "My journey kit" which is very helpful after diagnosis.
https://www.bcna.org.au/understanding-breast-cancer/Welcome to the site. The beginning is so hard and we are here to help. Kath x