A biopsy is when they remove something that looks suspicious. I've had bits of skin taken off for biopsies, a lumpectomy, which is an excisional biopsy of breast tissue, and polyps removed during colonscopy.
I don't think it spreads cancer, it would be more that if they find one thing that is malignant, there is probably more cancer around somewhere already.
I have been lucky in that no malignancies have been found. You may be lucky too. Or if they do find something, maybe it will be good that they found it early enough to treat.
It's never a good idea to avoid testing because "they might find something". You would still have it even if the doctors don't see it, and you lose valuable treatment time!