I'll offer a completely unsubstantiated hypothesis here. One of the most powerful defenses we have against viruses is a chain of molecules called the complement system. It's 9 enzymes that work together as a broad, generalized tool to fight off bacteria and viruses. The key component that works on viruses is ~10x more effective when you raise the body temperature about 2°C as happens with a fever, hence why you don't lower a fever unless it gets really extreme (>103-104°F). But there are alternative ways to achieve an increase in body temperature: sauna, exercise, etc, and both are seen as health-boosting activities.

Here's the connection. Nicotine is typically consumed by smoking/inhalation. You light tobacco on fire, transfer the nicotine to hot smoke, and inhale it. How hot is that smoke? I'm not sure, but I know it's more than 102°F. Not for long. The energy is rapidly transferred to the surfactant in the lungs. But if you're smoking a lot, you may be getting enough heat applied to the target tissues that respiratory viruses typically hit to enter the body and start infections. The warmer lung tissue may be slightly better at fending off these first-line attacks.

Again, this is a random guy on the internet spit-balling. It's a hypothesis. There's no data behind this, so don't start smoking thinking that it's healthy. It's not. The balance of effects is overwhelmingly against smoking. Don't smoke. If you do smoke, get help from a professional and quit.

But it is an interesting idea that's worth exploring in a world were people have flavored vapes with no nicotine in them at all. If that actually works and we can achieve the effect without those harms, it's certainly worth an introductory study.

Nicotine attaches to same receptors that the spike protein does, so smokers have less receptor sites available foe spike proteins to attack, hence the reason why nicotine users were less likely to get covid.