Quick demo I made using Unity 6 and the Unity terrain system. This is using one of my asset packages and 100% of the assets were made from scratch.
I must say that the Unity terrain system is not very flexible, you should look for another solution to scatter the objects if you want to optimize the maximum posible, but works good enough for this kind of demos.
Forest Package available here:
Unity Asset Store: https://prf.hn/click/camref:1011lfysW/destination:https://assetstore.unity.com/packages/3d/environments/mossy-cedrus-forest-267081
ArtStation: https://www.artstation.com/a/31751458
FAB: https://www.fab.com/listings/2b722914-7294-472d-9743-6a857aeb4685