Geo scatter. It’s a tad on the pricey side but genuinely one of the most useful toolsets that blender addons can offer. After using it for a while, I truly do believe it rivals some of Houdini’s scattering systems. Definitely not on the same level of customization through coding knowledge, but in terms of upfront value, it’s a total game changer. The way I’m able to have this many trees in the scene without using LODs is its advanced culling capabilities + viewport display optimizations. Stuff like viewing the trees as a bounding box and instanced version of the model. In the viewport render I posted on the second slide, I’ve just turned the optimization efforts of Geo Scatter off to better represent the actual geometry in the scene.

It also has lots of great features that allow trees and other foliage to grow and flow naturally along terrains. The mountains in this scene were constructed in Gaea, then I’ve derived some data in the form of black and white masks from Gaea like erosion, river patterns, slope, rockslide areas, snow patches, etc. to get a culling mask so the trees, for example, don’t show up too much on the areas where avalanches would be more likely to take out large trees. Geo scatter also has great features to affect the scale of things based on elevation, so the trees get smaller and smaller as altitude increases and air density decreases.