ZBrushCentral

I cant delete ZSpheres and how i can make hollow Objects

Hello, im a newbie with digital modelling, (im a classic dollmaker). My questions: how can i make hollow objects.
How can I make an Iris in an Eyeball? I mean a round Impression on a Sphere, i was trying to substract, but it does not work nothing.
Why i cant delete a Zshere? I was following a Tutorial, but it does not work with the Alt Key and klicking on the Zsphere. Can i modifiying the Marquettes in Zbrush Core2020? I can moving it around but i cant delete Zspheres. Even in my own made ZSphere Project i cant delete ZSpheres. As a newbie im sure im missing a lot of steps… Thanks for help!