You don't. If you live where cars are not needed, e.g. Tokyo, you'll just walk to your nearest small grocer and get the ingredients you need. That's what I did when I stayed in Japan for work.

The reason you haul entire shopping carts at once is because the trip to the grocery store is a big planned deal. That’s also the reason people buy bulk items and then let half of them expire.

The “ideal” for bikers and train riders would be easier, quicker trips to small stores to get ingredients for the next few days. I find I’m able to fit most of my needs into one pannier.

I've done that. You just bring something appropriate to carry it in.

Although now that I live closer to a smaller grocer, I just walk twice.

I mean the idea is that good urban planning would enable shorter and more frequent grocery store trips. Rather than a supercenter supplying everyone within 30 miles, requiring long drives, you'd have things distributed by need, i.e. general food stores every couple miles, more specialist places potentially farther away. Our current layout and shopping habits are contingent on car infrastructure and massive federal subsidies.

Would also decrease waste and increase general health, since fresher, less processed food could be purchased.

I will say that I've been able to bring 3-4 grocery bags onto a bus, which is enough to last me around 2 weeks. I've done this fairly consistently (basically whenever it's too cold/snowy to bike) for the last couple years. It might not be possible for a family without more than one person making the trip, but for an individual it can definitely work.

Three or four bags of groceries is totally doable on a bus or train.

This is ok though, going once per 14days for that 90% of stuff and having your car for that is ok. Otherwise if you run out of something, hop to your nearest store. Also here some of my friends and family are not reachable via public transport so I use car for that. But dont use it for commute every day, going to the beach/mountains every weekend, going to the store every other day, taking kids to school and back etc. For many this is completely doable but people are lazy

In civilized countries, it's common. Even on bicycles, by the way.

Buses where I live have a cargo rack at the front. If you had four bags of shopping (though that's really quite a lot - the bags are big) you would tie the tops closed and leave them in one of the racks until you reached your destination

yeah I do that all the time you bring a bag with you