Unless you are doing this, your builder will charge you for it, so Good design can still cost an arm and a leg to achieve for somone engaging a builder tobuild their home. The point is that there are many ways to achieve thermal efficiency without going down a route of some boutique German certification and likley cheaper.
Of course it can cost zero. The core fundamentals are easy to learn. Of course it will only get you so far without engaging an expert, but even shifting rooms and removing windows /reducing window size can achieve a lot. Sometimes a simple mirror of a plan would achieve a lot. All you would need is a builder willing to make changes (some aren't). It could even reduce the cost if you're removing windows or making them smaller.
If you are designing from the ground up, custom, and taking these things into consideration with your builder/architect, yes its "easy" to achieve. Buts its not $0. You're paying for that work and depending on who you go with it can be a massive premium. Just because you dont see it as a line item in your costings doesnt mean it was $0.
You're making a lotof assumptions, like substitutions to to window sizes can be achieved without custom manufacturing of the required sizes frames where a standard size substitution cant be made.
"The core fundamentals are easy to learn." is a pretty out of touch statement because while its easy to understand that something like a higher R value insulates better, understanding the real world value of those ratings across climates, materials application and installation methods is something professionals learn over years. Even BIM boy will shout BIM simulations from the rooftops on that one, and i'd agree. Its like saying Electrical work is easy to learn because fundamentally its just some wires that go here and here and don't forget to switch the mains off before you do the work. Or plumbing; water goes here and comes out there, hey presto, plumbing is easy.
Again I point you back to Ashington Homes, who is a builder by trade, has pointed out what it cost him, and it wasnt $0. Their point was they would spend another 30-40k doing additional work if budget allowed - which is probably an "at cost" figure given they'd likely be doing it themselves.
I'm not sure you've built a home before, but if you haven't, you're in for a bit of a surprise.