Here is a fisheye photo from the bottom of a rainforest. As you can see, some spots of sky are visible from the bottom. When you fly above the forest and LIDAR is looking down, similarly at some spots the LIDAR laser beams are able to hit all the way to the bottom at some spots through the canopy.

It's not a big fraction, but some of the laser beams are hitting the ground, and after data processing you filter them out and they five you the ground topography profile.

https://www.sciencephoto.com/media/972412/view/rainforest-ca...

Here is a side view that shows how the point cloud profile can look like. This is not from a rainforest but some other forest.

https://www.researchgate.net/figure/LiDAR-point-cloud-profil...