In flugplatz, the planes have detailed shadow. How is this done? Is there smaller brushes withing the plane or what? Because I want to add the same feature to the trees im using.

Clipbrush is making this shadow.

clip doesnt cast a shadow, as it doesnt calculate into the vis or lighting atall.
if you want your tree (i assume its a model, sprite or illusionary) to cast a shadow then you can make a block {or whatever shape} out of the NULL texture and float it above the tree, but this will block bullets etc
also the model entity should not be inside this block or it wont be there ingame
or you can then make that piece a func_wall or func_illusionary and set its flag to opaque
to clip a treetrunk i usually make a 4 sided spike, texture it null, make it a func_wall, set it to opaque, and voila, blocks bullets and casts a shadow, if you want the leafy part to shadow too then you'll need more brushes above the tree

-or if you want it to drop leaves when shot texture with the cr_matleaf {spelling?} and set the render amount to 0