I tried to get the image from [https://store.steampowered.com/steamos/buildyourown?l=english] but after clicking the link "Download the SteamOS installation" the parameter "ver" will be automatically changed from "custom" to "steamdeck".
The image that can be downloaded looks like a different version and not contain the "Automated install (WILL ERASE DISK!)" or "Expert install" steps.

Is there any trick, hidden menu point or different image that can be used to install Steam OS?