I once did expect supermarkets to sell reasonably fresh produce, but I've since learned that thanks to their use of cool and cold rooms, produce can be several years old when it hits the shelves. I expect growers markets to sell the freshest produce as they generally can't afford the kind of strict temperature control that supermarket chains can, and independent grocers to fall somewhere in the middle ground. You also missed the many local food hubs and produce shares, which are more and more becoming a valid source of fresh produce.
When you ask about particular fruits and vegetables, you don't specify if you mean supermarket, IG, or farmers market / farmer direct. In supermarkets, all produce seems to be disappointing at particular times of the year, in IGs that happens with items like onions some of the time, and at FM/FD they just aren't available all year round unless in season so they're generally always good when available. Also, comparatively, supermarkets have the most uniform but bland produce, IGs have a more flavourful but variable range, and FM/FD has the most variability but is generally the best, and the produce lasts best of the three.
Supermarket fruit and vegetables are bought from the grower at a less ripe stage and that allows them to be retarded for years, but misses out on delivering the full flavours and nutrients as many of these aren't developed until full ripeness, and what little is there to begin with, is consumed by the produce during storage to bring it to ripeness.
Quality/ripeness/freshness issue: As above. How can anything that isn't allowed to ripen on the plant absorb all the extra nutrients that it would have gotten with another week or two on the plant? And how can this unripe produce then become "ripe" without any further nutrients? Obviously that produce is going to need to consume part of its own nutrients to complete the ripening process, nutrients that in a normal (IG, FM/FD) setting it would have obtained from the parent plant?
In general, the nutrient density of supermarket produce is uniform, but lower than real fresh produce. Because some supermarket produce is kept for long periods, some deterioration occurs and when you take that produce home, it spoils far more quickly than fresh produce.
Oh - also look into meat storage, and age at time of sale. Supermarkets apparently can, under strict temperature control conditions, buy meat at today's prices, freeze it for years (up to thirty was a figure I've often come across) and then sell it at future prices in the future. Even now some meat can probably be several years old before it hits the meat section styrofoam. Okay, this is mostly hearsay, but there's been a well-documented case in China of forty(!!!) year old meat still being sold, often transported without refrigeration, and if they with lmited refrigeration and temperature control can keep meat for forty years then you might reasonably suspect that supermarket chains have been doing much the same but with far better technology...
That's about all I can put into a brief comment, hope it helps.
