Produce continues changing after it is picked, and some of its nutrients decline while others do not move at all. The distinction depends on the chemistry of the specific compound rather than on freshness in general.
Minerals stay, some vitamins do not
Minerals such as iron, calcium and potassium are elements and cannot be destroyed by storage, light or cooking, though they can leach into cooking water.
Vitamins are complex molecules and several of them break down over time or under specific conditions, which is where losses actually occur.
Vitamin C and several B vitamins are the most fragile, being both water-soluble and readily degraded, while fat-soluble vitamins are considerably more stable.
The plant is still alive and using its own reserves
Harvested vegetables continue respiring, consuming sugars and breaking down compounds as their cells run down their stores.
Leafy greens with a large surface area and high metabolic rate decline fastest, while dense roots and tubers change slowly over weeks.
Cold slows this dramatically, which is why the temperature history of produce between field and shelf affects it more than the number of days elapsed.
Cutting starts a different clock
Slicing ruptures cells and brings enzymes into contact with compounds they were previously separated from, which begins degradation and browning.
The exposed surfaces also lose moisture and meet oxygen across a far larger area than an intact vegetable presents.
Prepared and pre-cut produce is therefore on a shorter timeline than whole produce, regardless of how it is packaged.
Freezing preserves more than its reputation suggests
Vegetables destined for freezing are usually processed within hours of harvest and briefly blanched, which halts the enzymes driving deterioration.
They then sit at a temperature where chemical change is very slow, so their vitamin content can compare favourably with produce that travelled and sat on a shelf.
Texture suffers because ice crystals damage cell walls, but that is a separate matter from nutritional content.
Cooking moves nutrients as much as it destroys them
Water-soluble vitamins leave the food and enter the cooking liquid, so boiling and discarding the water removes more than steaming does.
Heat also makes some compounds more available by breaking down cell walls, which is why certain nutrients are better absorbed from cooked vegetables than raw ones.
The direction of the effect depends on the nutrient and the method, which is why variety across both raw and cooked preparations is the practical answer rather than any single rule.