No Stupid Questions

If you smoke, quitting is the number one thing you can do to improve your physical and mental health. In the US you can call 1-800-QUIT-NOW or visit Lung.org for free evidence-based resources to help with quitting.

On to the OP question, inhaling smoke of any kind exposes you to carcinogens. In tobacco smoke, the most abundant by mass is Benzo[a]pyrene, which is created by the process of burning. It is absorbed into your bloodstream in the lungs and damages DNA in every cell it touches. If the DNA damage occurs in the wrong gene, that can kick off a decades long process of more mutations that ultimately lead to uncontrolled cell grow. Inflammation, also caused by inhaling smoke, encourages cells to grow more and amplifies cancerous mutations.

Quitting stops the exposure to new DNA damage and inflammation, but there has already been some damage done.

Edit, adding an alcohol answer: Heavy alcohol use also takes a toll on your body. A big cause of this is that alcohol is fairly quickly metabolised into acetaldehyde by the liver. However, the acetaldehyde dehydrogenase enzyme can be limiting to detoxify the acetaldehyde. Acetaldehyde is an aldehyde, much like formaldehyde, and thus highly reactive with proteins and DNA. So it can damage those as well. Acetaldehyde is also the chemical mostly responsible for feelings of being hung over (in addition to general dehydration).

The effect is cumulative, you'll do better if you quit than if you don't, but some damage has been done. It's a "the best time to plant a tree was 30 years ago, the second best time is today" situation.

Because the risk doesn’t stop immediately after you quit smoking or drinking. A lot of the DNA damage that causes lung cancer is already done. Premalignant cells may have already formed and can still become malignant even after one quits smoking, or the tumor may have already formed but be slow-growing and asymptomatic.

The lung cancer risk does go down slowly year after year for smokers who quit. So people who keep smoking are still much more at risk.

And then of course there is a baseline risk of cancer and even people who never smoked can get lung cancer, especially those who live in cities with a lot of smog.

Damage to cells is already done

Still a (much) lower percentage than smokers.

Yes you can ironically get lung cancer after you've quit smoking. But more people stay healthy for longer when they quit.

And yes high alcohol consumption is devastating. Any amount is carcinogenic, there is no safe lower limit. Drinking heavily as an adult is a serious health risk, with lasting impact. Not drinking is always healthier than drinking. Quitting is always beneficial.

The stories of people dying of lung cancer after quitting and liver failure after quitting drinking, are mostly told by people that smoke and drink, as an excuse to not have to quit.