Seems like an unusual question.

Private aircraft do not park at a terminal; they park at an FBO. At YYZ two FBOs are Landmark and Skyservice. The operator will choose based on fuel price and services available. Customs needs to be prearranged and an officer will come to meet the airplane at the prearranged arrival time. If your operator has Corporate CANPASS and the pax are eligible you may not even need to talk to a customs officer. Just step out of the plane and into the limo. In either event it's very quick and painless.

Most of my travel is by private plane. It's been years since I've actually seen a customs officer upon arrival into Canada.

For a private flight into the Toronto area YYZ would be my last choice