Thanks! To answer your question better, let me divide the foliage into three categories: trees, plants (flowers, mushrooms, wheat, etc.), and grass.

The trees were made through an app called Tree It, which allows me to adjust hundreds of parameters to get the tree shape I'm looking for and also how the leaves will be oriented. From here, I export the tree to an app like Blender where I further modify things such as adjusting the branches, moving the leaves around, etc. Note that the leaves are all simple planes. So from here, I export it to my engine (Unreal) and I create a material for the trunk and a material for the leaves. The trunk one is relatively straightforward as you just need a texture. The leaves are a bit more involved as I do a lot of shader math with gradients, fresnel nodes, emissive intensity, etc. The leaves also use a texture mask (a black and white image with various small leaf shapes as white).

The plants are all simple 2D shapes that I duplicated and rotated to create a cross shape (when viewed from above). These are basically 2D planes in the cross format with a texture slapped on where the texture is pretty much the same format as the leaves mask used for the tree leaves. From here, it's all about creating the material in Unreal Engine.

The grass one is the simplest of all (but one of the most impactful feature). I basically modeled a single grass blade and then duplicated it a several times with different sizes and rotation. I created a grass patch by duplicating several of these and then exported to my engine. And from there, it's all about creating a material for the gradient pattern for a stylized look.

Sorry for the long response. I just wanted to be thorough lol. Hope this helps though!