They are typical as low-point drains and manifold drains, both on the main line (always charged). Don’t just cap it if that would make a dead end, especially if it’s sloped down to drain. I can’t tell very well from the picture, but if it’s on a tee you would be okay putting in a threaded cap.

“king drains” or auto drains are common. A short nipple and either a brass cross-top or pvc ball valve would be better. Just be sure that it is accessible in the end.

Hollow doors have about a 1" frame of solid wood around the perimeter.

If the door frame is not currently falling out of the opening there’s enough meat in the opening to mount a new one. Fondots is right. Re mortising well for the hinges and modifying the door to fit will be more effort than getting a pre-hung.

Picture looks like real wood and not paper, hardie or vinyl. Venat0r has the right idea of taking a sample in to your local yard. A couple possibilities I came across online are https://www.becn.com/products/siding/wood-siding/C-503734 and https://truwoodsiding.com/products/1-2-cedar-shake-lap-siding/ although the online pictures are garbage for them.