Like the Nexus 6, there are no visible screws on the rear cover. Unlike the Nexus 6, this is a unibody device, so there won't be any handy peel-off rear cover.
The 6P's smooth enclosure is broken only by this small, suspicious plastic panel at the bottom. Let's pile a hot iOpener on it and see what's inside.
With enough heat, the panel is easily pried and peeled off, exposing four screws.