Boulders will generate made of either granite, diorite, or Andesite. Granite boulders will generate very wide and will never get above 5 blocks tall. For andesite boulders, they will generate much taller instead of wide, they will be able to get to 7 blocks tall at the max. Diorite boulders will be relatively small never getting above 4 blocks tall. Inside of the boulders, many different ores can be found such as iron, coal, or copper.
The new trees will get much taller than the ones we currently have in game. The new trees can be 9 to 15 blocks tall (excluding the leaves). And a new thing only birch leaves will do is that leaves can fall from the leaf block and make birch leaf carpets for new natural decoration.
I made a world with custom structures and other things. The island that I am using is mostly a birch forest. I added a few more fallen logs alongside the natural ones (This is on the switch) with mushrooms on top of them. I also added two versions of a custom structure that mostly resemble a small circle of ancient pillars. Some pillars are collapsed and are represented by stone brick slabs. The center has a zombie villager spawner. These ruins have a few ores scattered as parts of a pillar or in the ground with the ruins. You can find iron, copper, emeralds, or even a diamond ore. These ruins would be an important early-game structure with the ores.