3D Tilemaps ( Godot 4.1 )
https://www.youtube.com/watch?v=JxbnStn-BIY

Designing 3D Game Levels from Scratch
https://www.youtube.com/watch?v=dV6S_11pyzo

See also free grass plugin
Otherwise, creating grass from scratch:
https://youtu.be/kGtSHFeMqTo?si=zX_GgeEmdCNHPJ24

For lots of same objects like trees, consider using MultiMeshInstance3D:
https://docs.godotengine.org/en/4.2/tutorials/3d/using_multi_mesh_instance.html