Voxel Builder Add-on 1.21+

Create detailed structures with the mod Voxel Builder, which allows you to build structures with pixel precision within a single block. Use platforms of different sizes (8x8, 12x12, 16x16) and add blocks, including items from other add-ons. The assembly is perfect for creating miniature detailed objects in Minecraft.

How does it work?
First, you need to place one of the Voxel Layers in your inventory
When you place a Layer on a block, a Grass cover will appear
Now you can place blocks inside the block

Recommendations
FOV at 50 or lower
Reducing camera sensitivity can increase block picking accuracy
Some blocks, like plants, may not match the original and will look different when placed
Mobs cannot be placed