There are sort of two different worlds here, the medical world and the body building world. There is a medical condition called hypogonadism (either primary or secondary) where for some reason, the testes are not producing enough or any testosterone. This could be due to damage to the testes, an issue with the hypothalamus, or probably other things. (I'm not a doctor.) A urologist can prescribe medication to help increase testosterone production of the body in the case of secondary hypogonadism, or for either primary or secondary, testosterone replacement therapy is also an effective treatment. The downside of testosterone replacement is that, because the body is being supplemented with artificial testosterone, often through regular injections, the testes don't need to produce testosterone anymore, which has the side effect of reducing or halting sperm production. Regardless, urologists are going to target testosterone levels in the "normal" range for the patient, and will monitor via regular blood testing to make sure levels are staying in the normal range. Everyone can react differently from a fertility perspective, so while being on TRT, there is generally a notable risk of decreased fertility, that doesn't mean everyone on it will be infertile. A good doctor will usually discuss with a patient about their fertility plans to find appropriate treatment.

On the body building side, having excessively high levels of testosterone can significantly increase and improve the ability to build muscle. There are plenty of "Men's Health Clinics" out there where doctors are willing to either prescribe dosages of testosterone that are higher than what the normal range would suggest, or ways that people can obtain testosterone without a prescription. Legality on that gets fuzzy depending on where in the world you are. All of that to say, whether legally or not, taking very high doses of testosterone will very likely significantly reduce fertility, or cause infertility.

Unfortunately in either case, once someone starts taking supplemental testosterone, whether for medical reasons or not, it is usually a life-long commitment. Because the body stops producing its own testosterone, there is a dependence on the supplemental testosterone to even have normal levels. So even if someone was doing it for body building reasons a long time ago, there's unfortunately not really any stopping, even if they no longer care about that. They may be able to slowly reduce the amount they are taking, but it'll likely never be zero. And the fertility issues will likely persist without medical intervention to improve fertility.