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.
A bigger factor is that the Canadian prohibition was only controlled at the federal level in the first place, like all Canadian criminal law, so only the federal government had to legislate to change it. The provinces have however done lots of subsequent legislation to regulate the details (e.g. distribution channels and the exact minimum age limit) in a wide variety of ways.