That's not to say plumbing doesn't take skill, it certainly does, but the point of it is that nobody except the next plumber cares how the pipes are laid out, so long as it works and works well. It's when one blows and you have to fix it, or install a second bathroom, that shit really tends to come out. If I'm the one that has to do the work, I can only hope the previous plumber had some idea of what they were doing, and didn't just leave it entirely to automation.
But if they did I'd hope they trust, but verify.