The 2d sky material differs from the normal sky material in that it won't make the compiler flag the visleaf as showing the 3d sky. However, for this to work it requires that all visible sky faces in the visleaf use the 2d sky.
If you can convince the server admin to list the texture file as a required download. Or if you pack the texture into the bsp file, that's what most people do.
Check out the tf_logic_robot_destruction, tf_robot_destruction_spawn_group and tf_robot_destruction_robot_spawn entities. And of course, the trigger_rd_vault_trigger entity too.
Normally you'd just be able to use the existing materials, but the shader used is likely not suitable for what you want, so you will have to craft material files of your own. But that's little more than a tiny boilerplate textfile in most cases.
If this is your first time logging in after the migration (Feb 8, 2022), you must reset your password to log in. Follow this guide if you're having trouble