In 2018, Canada's federal government passed the Cannabis Act, which legalized the sale of various cannabis products from government-sanctioned sources and allowed adults to possess up to 30g in a public place. In Québec, this 'sanctioned source' is the Société Québécoise du Cannabis (www.sqdc.ca), which lists its stores and their hours, as well as instructions for online purchases, on its website.
You're not allowed to smoke or vape cannabis anywhere that smoking tobacco is prohibited, which includes restaurants, bars and most public buildings.
It's illegal to drive while under the influence of cannabis anywhere in the country. Québec has specifically banned vehicle use 'if there is a detectable presence of cannabis or other drug in the (driver's) saliva.'
While in most provinces, the legal age to purchase cannabis is 19, in Québec (and Alberta), the legal age was initially set at 18. However, at the time of research, the Québec government had proposed raising the legal age to 21.
You can get an update on cannabis rules in Québec from this government website: https://encadrementcannabis.gouv.qc.ca.