How are holding tanks plumbed

Looks like I might have a holding tank leak. Will check it our tomorrow. But right now Iam wondering how they put these things in. Iam guessing they mount the tanks then once the floors are on, cut a hole in the floor for the drain and continue drilling into the tank. Then stick the drain pipe in. But there must be sone type of fitting or gasket around the drain pipe?
Thinking that Some on here have toured factories and might know the answer or have worked on holding tanks before.