Well, after an intensive internet search it seems that the google camera app availability depends on your device,your android version, and maybe even your country.
The app that I am looking for is offered for example by apkmirror.com (good source?),
see Google Camera APKs - APKMirror
On my devices with android 4.x or 5.x it seems to need not the latest app version, but version 2.xx, because app version 3.xxx needs Android 6.x
I will install that app manually and give it a try, and report back here.
Anyone other with experience / feedback?