Sometimes they are. My house is single story and has sewage pipes in the walls. Regardless of whether that is a requirement, the pipes have to go through some part of the structure to get to the outside, regardless of whether that's a wall, floor, foundation, etc.

It’s possible but only done for convenience where it makes sense to, not a requirement for new construction since you just put the pipes in before the pad gets poured.

Your toilet pipes are inside a wall? That’s pretty strange, toilet drain pipe is usually 3” dia. Are there 90 degree bends? I don’t understand how or why you’d put a toilet drain pipe in a wall when the drain pipe is in the middle of a bathroom floor, not in a wall.

Are you sure you aren’t thinking of sink drains?

Yes but GP said large pipes, e.g. for a toilet or the main pipe that connects to the municipal sewer. In a single-story house these would be under the floor. Pipes in the walls of a single-story house would be sink drains or vent pipes, and are narrower.