This snapshot, 21w07a (the first one released in the seventh week of 2021, hence the name), focuses on two main features.
The first is the addition of grimstone, a variant of stone that generates at the very bottom of the world for the last 64 blocks.
Grimstone comes in over a dozen different variants, including slabs, stairs, tiles, bricks, walls, and more.
The block is slightly harder than regular stone, making it take twice as long to mine.
The second main change in 21w07a is revamped ore distribution. With the increased world height, the placement of ores in the world has been tweaked to balance out the game.
Diamond now generates much more prevalently at the very bottom of the world, alongside redstone, with streaks of gold found there as well.
Lapis lazuli now generates nearly everywhere underground, and emeralds are now very common on the tops of mountains in that biome.
See the image below for a detailed diagram of the new ore distribution.
Cave generation has also been tweaked, reducing the number of caves and prevalence of flooding.
This snapshot was meant to include new grimstone ores, but these textures have been left out.
Other changes in this snapshot include a reordering of the inventory tab for redstone, moving commonly-used blocks and items to the top, and the addition of chains to hold up mineshaft passageways.
For a visual overview of this snapshot, see my video below: