This "drug" is a weakened form of the bacterium, which apparently stimulates immune response. So I guess it works for both TB and bladder cancer just by getting your immune system to notice something is amiss?

I can explain. BCG infects the actual epithelial cancer cells inside the bladder, triggering Th-1 response (production and release of cytokines by activated CD4 T cells).

The cytokines induce an inflammatory response, which I turn activates other immune system cells such as CD4 and CD8, NK cells and macrophages.

The immune cells then attack the bladder cancer cells, hopefully destroying them, thus "fighting cancer".

Source: Li J et al, NPJ Vaccines. 2021;6:14.

Turning it off and then on again works in a lot of surprising places