This vertical metal pipe in the backyard? old well? septic

This is the wrong place to save a few bucks on the pipe - you may live to regret (and smell) it.

SDR 35 (not what you linked to) may (IIRC) be considered acceptable, but it's foolish to use it on the inlet side .vs. schedule 40. Likewise, you may only be required to use 3" pipe, but 4" is well-worth the minor cost increment in the lack of trouble it will assure you of for the relatively short run from the house into the tank. If you are using a PVC Tee inlet baffle rather than a cast-in-place baffle, that should be a 6x4X6 tee with appropriate 6" pipe extension for the lower leg. It's acceptable to exceed requirements, and doing so can save trouble later on.

A lot of the other pipe is only acceptable for the drain-feild side of the system, distributing the treated wastewater. In my opinion and my septic system, it's schedule 40 all the way to the distribution box (beyond the tank outlet.) Cheesy pipe can be used in the drainfield.

I finished digging it up. It was just an oval piece of concrete (about 3 inch deep) that had the edges wrapped with metal. 0_0 so I guess trash is the closest answer. Wish I knew what it was t here for...it seemed deliberately placed.