The optimal masonry? Ytong or sand-lime brick?

So stupid question, where can I find the Steinemantra? The search function only brings up posts where it is mentioned, but not linked...

You will search in vain here for external links, but there is a hint, and I have at least explained it summarized here:
https://www.hausbau-forum.de/threads/blaehton-vor-und-nachteil-dicke-anbieter.44819/post-614045
https://www.hausbau-forum.de/threads/ist-dieset-aufbau-des-mauerwerk-fuer-kfw55-ausreichend.42439/post-553805
https://www.hausbau-forum.de/threads/grundriss-winkelbungalow-150m-mit-erweiterungsmoeglichkeiten.42546/post-557361
https://www.hausbau-forum.de/threads/aussenwand-kalksand-polystyrol-vs-hochlochziegel.42764/post-561076
https://www.hausbau-forum.de/threads/aussenwand-fuer-kfw-40-mit-wdvs-oder-nicht.38171/post-473433

Thank you. I'll take a look.

If you want to have Ytong built, go ahead. If you want a different masonry, find the general contractor who only does that, but don’t force any general contractor to deviate from their standard

I can only agree with that. If you don't like the construction method, for whatever reason, you are free to build with someone else. Every general contractor has their preferred construction method and their subcontractors as well as masons who are very familiar with their stone or construction method. If someone can't work with other methods or doesn't have the sources to get the material cheaply, then the quality can be rubbish or it can be more expensive.