@LaVitesse2022 - no car here, have an older child (3, doesn't like walking/other modes of propulsion). We are on London, maybe 20 meters from the entrance of a tube station and 8-14 minutes from busses that go other places. We have always primarily used a pram/buggy. I haul the pram, with my daughter in it, up and down the stairs of the tube all the damn time. Like, probably 12 staircases every weekend. For this, I strongly recommend a yo-yo. It is light, you can carry it yourself if you are relatively strong. It is also narrow enough to go on escalators and not block the walking half. I must say that because of the pandemic I never took baby with bassinet on the tube, but I am willing to bet I would have done the same, so long as the bassinet felt securely attached. Yo-yos are fine to navigate on the bus, though sometimes the gap is a bit big to get on, but again, then you life the whole thing up, baby and all, and put it on the bus. We walk miles with that thing. It is not the most sturdy pram out there, but in my opinion it does a very good job for city life.
You can get little fold up travel car seats when they get older (I don't know exactly, but I reckon less than 6 months probably wouldn't work for little fold up guys) and we use those when we travel, but I cannot underscore strongly enough how infrequently we drive, and how this infrequency contributes to my relative ambivalence about whether they are the safest thing to use. When we go to my in-laws (a few times a year, extremely car oriented society) she has a proper car seat, partly because they would not hear otherwise, and partly because she has a same aged niece who lives next door to the parents in law. When we have visited my parents (once per year), again, very car oriented place, I think my mom has borrowed a car seat from a neighbour.
We borrowed a newborn baby car seat from a friend. I would have otherwise bought a used one from a source I felt was fairly reputable (someone I work with, someone at the nursery) because, given how infrequently we used it, say, 4 times driving slowly on London, that risk profile was fine for me. I would feel differently if I drove even, say, monthly, with a baby in the car.
I think the best way to think about it is to really assess how you live your life, how often you go places, how muddy the places you go are (yo-yo wheels suck at mud), what options you have for borrowing car seats, and then make a decision from there. Your mobility system could look really, really different from other peoples/advice online, and that is fine! Make sure and the way you think about storage, ease of clipping a car seat in, robustness of pram, size of pram, weight of pram, agrees with the life you actually live. Also, you can legally take a baby in pram in a black cab without a car seat if the only thing you are worried about us getting home from a hospital. I hope that helps!