It took me a long time when I first started living alone and taking care of all my meals to figure out how to not waste my food, mostly through trial, error, and taking note of every single useful bit of advice I came across online or in meatspace.
I still don’t think I’m particularly efficient, just that I waste a lot less than I used to, and if I could fit a freezer chest in my apartment, I know a few ways that I could waste even less. Mainly what I do now is to shop at a local grocery just about every day, I’m blessed with an abundance of those and good options, and try not to buy more than I will cook in a day or two. This has become easier since switching to an almost entirely meat, egg and vegetable diet so I don’t have dairy products, soy products, bread, pasta, cereals, noodles, beans, or anything besides my spices, dried herbs, salts, oils and vinegars sitting around. Despite that, I still end up throwing away more than I would like every few weeks, but probably not 2kg a week.
Had I grown up in a household with people that knew how to manage a household, it would have been a lot less trial and error, but with our over reliance on packaged foods instead of just what you can find in the produce section and at the butcher, probably a lot of people like me are growing up not learning how to manage their pantries and fridges.
That’s without even factoring in restaurants. It isn’t uncommon for people to not finish their food and leave it rather than taking the rest with them, and more so the more people eat out. Rather than finishing their leftovers at home, if they even took it home, people will often just go out to eat again the next day.