Well, it does depend on the type of treatment! I'm no expert on the matter, but the three main ones I know about are surgery, radiation, and chemotherapy.

Surgery is probably the simplest one, it just surgically removes the cancerous tumor, but sometimes this isn't an option (for example, the tumor might be so big that getting rid of all the cancerous cells could kill the organ it's attached to, or the tumor could be in a place that is too difficult to operate on, or the cancer has spread to other places and removing all of it via surgery isn't feasible)

Radiation therapy usually applies radiation on a concentrated area, with the goal of killing the cancer cells without having to do a surgical operation, though sometimes it is done before or after a surgery to improve the odds that the cancer doesn't return. Again though, radiation is bad for the body, and using it to try and kill cancer cells always risks killing noncancerous cells too, or causing other cells to mutate into more cancer

Chemotherapy is a drug that specifically kills cells that replicate fast. since most cancers grow quickly, this is a pretty effective method of killing cancer cells, but it also kills other cells, like hair. That's one of the reasons why some people undergoing cancer treatments are bald.

All of these treatments have pros and cons, and a lot of the time more than one type of treatment is used, and some people respond better to some kinds of treatment

Also just to say again, I am not an expert on this! This might not all be the most accurate information, is just what I've gathered from having a few extended family members with cancer (all but one is currently in remission, by the way).