Usually the cancer cells are throughout the body, even if the tumors themselves have not spread. This is why you might cut out a solid tumor, and then give chemo
Usually the cancer cells are concentrated where the tumor is. One of the first things they may do upon diagnosis of cancer is a PET scan (which shows you where cancerous stuff is throughout your body).
Life advice for all the young folks: don't get cancer.
metastatic cancer is a numbers game. for example. at stage 0-1, you might still have millions of cancer cells throughout your body, and there is a good chance your immune system can clean them up. At stages 2 or 3 there might be trillions of non-local cancer cells, with a proportionally greater chance of propagation.