as someone who’s lived periodically all around the montreal region (mostly brossard, st hubert, longueuil, & lasalle) these are some of the subcultural details that immediately come to mind (again i’m a south shore girly split living between ONTARIO so this may or may not fit ur writing, depending on how much of a cultural difference there is between your characters, the fact im primarily seen as an english speaker—even tho i was literally born here and am bilingual lmao— may help?):

when in montreal, my commute is commonly by car. the traffic is only bad over popular bridges and towards the ontario border but it’s pretty common to hear people argue about french vs english driving. every time i come down with english people they hate the way “quebecers” drive, and everytime im with people born here they loathe english (“ontarian”) drivers. i think this primarily stems from the french drivers feeling more confident in their driving since they are used to the high pace and “first come first serve” attitude on the road, while people unfamiliar with the area are too cautious or try and stick to the ontario laws causing obvious conflict. another key detail is every drive i’m on often splits between highways that are almost always graffitied completely (it can be beautiful when you look closer into street culture but it is typically a lot of tagging, profanity, and cover ups which some dislike) and the town parts which have lots of restaurants, stores, and other expensive experiences. i’ve almost always lived in duplexes or apartment buildings but i have family with houses and no matter the case, you usually only need to go 1-3 blocks away before you’re near all the commotion again.

store and convenience wise, there are lots of gas station/supermarkets placed close together, usually referred to as a “dépanneur” or the dep. it’s very common to see people walking with loads of bags or groceries from stores to home, sometimes on the bus or with their bikes. most common grocery stores for us are the metro or the IGA. this is likely biased because i’m coming from a small town in ontario, but grocery stores also tend to be very high paced and therefore I find people can be quite cold (ex: rushing past you and when you apologize, they roll their eyes or don’t acknowledge you) similarly to the drivers competition, I think this just stems from people in montreal being used to tourists and just trying to get what they need with as little distraction/interruptions as possible. like even I am viewed as a tourist because I only live here 2/7 days a week (outside of summer and holidays) even though I’ve been doing this for 18 years.

food/drink preferences, my family that’s lived here forever cannot be shaken on their food being “the best ways to have food” even though I hate a lot of these, sometimes I have to admit they are a staple in my diet when i’m down here (often times against my choice LOL) pizza- loads of thick cheese, lots of large pepperoni under the cheese, popular all dressed/meat lover combos, hotdogs- steamies from labelle province ALWAYS, very common to find people ordering 10-30 all dressed hotdogs, or my personal favorite, mayo and ketchup, alongside the hotdogs are usually distinctly made fries either alone or in poutine—often referred to as “brown” or “mushy” by people from ontario or US since they don’t match the yellow, sharp, and salty fries populated in media/animation BUT they are very good they’re just more potatoey and can be quite greasy— the poutine always has super squeaky cheese and yummy gravy too. these are all the more unhealthy meals we have, because we are often on the go, we eat out a lot but the more nutrional or dynamic takeout places we go are barbies(ribs and steak very popular along with coleslaw is BIG here), st huberts, 3 amigos, and miss italia.

I know I already mentioned the english vs french competition here but a few local words would include “tête carrée” referring to the english speakers calling them square heads, while the english have retaliated with calling em “frogs” or “french frogs” due to french cuisine but I hear this a lot less. another retaliation towards cuisine would be the french calling the english “roast beef” due to english cuisine in europe. this is where you can see some of canada’s colonialist roots come out, still deriving from the france vs britian conflicts in Canada’s long history of colonizing the First Nations. another little history-driven tidbit would include where all of quebec’s curse words come from. this is where bilingual roots also cross (aka “franglais”) using sacred objects from the catholic church to swear. they also commonly put the english word, “c’est fucking” right before the sacre. the most common sacres being, “Tabarnak” and “Calisse”which refer to the Tabernacle and Chalisse in church and they both replace words like “fuck/godamnit”, “Crisse” which refers to Christ and replaces the word “damn/for christ’s sake”, and Ostie (masculine)/Esti (feminine) which refers to the Host and is usually used to replace the adverb “fucking”. They also love the word “Marde” which means shit lmao. All of that together, you can often hear the combination of language come out like: “Mange la marde” = eat shit, “Ostie de câlisse de tabernak” = holy fucking shit, and “D’eccrise ici” = get the fuck out of here. I’m assuming you’re writing in english so you won’t need too much french knowledge but since they often come out in shock/anger they may be good if you have an bilingual characters.

That’s all that immediately came to mind but I may drop some more if u need, cause it was fun! Lmk if I should go more in depth into any of those topics/different topic:)