I want to know what is the difference between mammogram and ultrasound when it comes to breast cancer detection?

"Apples and Oranges": A mammogram is an excellent (and inexpensive) screening test, meaning that it is designed to find cancers too small to detect by palpation. In contrast, ultrasounds are diagnostic tests, meaning they are helpful to determine whether an abnormality seen on a mammogram (or exam) is worthy of a biopsy. For example, one can only distinguish a cyst from a tumor by ultrasound.

Xrays vs ultrasound : Mammograms are images obtained by compressing the breast and exposing the breast tissue to x rays from several directions. Various findings on these X-ray images could be a sign of cancer. Ultrasound images are obtained by sending high frequency sound waves into (breast) tissue and capturing reflected sound waves back. These are then used to generate an image. Breast cancer can often be detected.

X-ray mammograms: Rely on difference between tumor and tissue (not there in the 15% with inv lobular ca until quite large), requires compression (universally hated), and leads to u/s for biopsy direction, and sometimes mr. 3 tests: one relatively insensitive, one not reliably reproducible, 1 fraught with false positive.