I agree, I SHOULD of dug a test hole.
As to what all was done when the guys dug.
They just dug it and covered it, about 12" deep, about 8" wide.
Laying the pipe, setting the connections, setting the heads is a separate cost estimate in my "just learning how to do this" phase. The guys I can get to dig, I don't trust to make good connections or set the heads. Buying a machine is in the far future. I doubt if vibrating plows can be rented locally though I have not looked. ( nor do I know how to use one) I don't know about "ditch witches" I don't know about the cost and technique of mechanized digging. I have personally never seen it done locally. I do have a lot to learn. When CaptSteamRotor said, " we charge 1.50 a foot with the 255 to install 1 inch NSF poly 8 inches in the ground" , is a good starting point for me.