Here in Ontario Canada I can walk into a local neighbourhood cannabis store (one of many on every block, it seems) and make a purchase using my debit or credit card. I'm not sure if any of them even keep a cash float although I imagine they must, just in case granny comes in "for medicinal use". Alternatively, I could just go to the government-run web store and get home delivery through Canada Post at no extra charge.
What is going on is that not that weed is 'half-legal' in the states. It is fully illegal. What is true is that the federal enforcers have more or less decided to leave people alone when the state allows the use of Marijuana. Pre 2017, the exact same thing was happening across Canada where local jurisdictions allowed cannabis use and sales, and the RCMP basically turned a blind eye. Vancouver is the most obvious example, where there was actually a decline in the number of dispensaries after weed became federally legal.
That's not what supersede means.
There is no federal law about wood burning stoves because the constitution assigns environment to the provinces.
AFAICT it’s the federal laws about pollution.