As an example, diamonds generate in that triangular pattern between y-144 and y16. Since the Minecraft world ends at y-64, you won’t be able to reach those juicy diamonds spawning below the bedrock. That means,simply, that you’ll find more diamonds the deeper you go, as the lowest levels get you closest to the centre of the distribution pattern.
Previously, most types of Minecraft ore would generate evenly regardless of the y coordinate, so if you were mining within a given ore’s height range, you had an equal chance of finding it. Now most ore has a sort of ‘triangular’ distribution pattern, meaning that you’re most likely to mine successfully toward the center of the height range.
The chart below, courtesy of game designer Henrik Kniberg on Twitter, lays it all out. Wherever the ore distribution is at its widest is where the ore is most likely to spawn. But bear in mind that ore can’t spawn in open air, so the upper-level iron distribution isn’t quite as generous as it might seem on the chart.