Maybe this is not he correct site but how do you tell people to leave my cooking alone.
Unless it is a big complex meal there is rarely a need for more than one chef. Others can assist with cutting and cleaning but one person can handle 4 burners. In another kitchen as a guest I always ask what I can cut or prep but I stay off the stove.
I am a better cook than my mom. I worked as a short order cook for 7 years. For example over the holidays she will ask me to cook breakfast and I will start sausage on low while I prep other stuff like cut hash browns. Mom will come in the kitchen and turn up the burner (often when I am not looking and I get blamed for burnt sausage but pink in the middle). It is her physical kitchen but she asked me to cook.
Or BBQ for guests and they will open the grill and turn the meat (I bought). Or I am smoking and they will check on the meat which loses 15 minutes of heat and then ask when will it be done.
How to politely but firmly tell people to leave my kitchen alone?
If it's not your kitchen, you really can't tell them to leave it alone. My approach is to save the "serious" cooking for when I am alone, or in my own home cooking for my family. When I am with extended family, or at a friends house, I try to think of cooking as more of a social activity. Give people easy jobs. This politely clarifies roles and ensures anyone who wants to be involved can be. For the smoking, maybe a small sign: "Please ask me why you should not open the smoker."