Trees like in City Skylines

How are everyone? I was struggling with a project on Unity and I have a task to create a terrain with a lot of trees. It is top view (angle from 90 degree to 45) like RTS game. I saw what I needed in City Skylines but I couldn't find any details. I know that you can create around 200k trees in city skylines and with special mode around 1m. I checked out lods, static batching, imposters, instancing, critias tree system from asset store and also tried to combinedit but either quality was not good enough or amount of trees too small. Any ideas how it can be created? If it is few different techniques how it was combined?