While writing, does Zettlr have an auto-save feature?
And I noticed, when I manually save (Ctrl+ S) while the doc is open, the undo history is reset - is that correct?
… the Undo-history is reset? That should not be happening. I keep the Ctrl+S-function capable, because it gives an additional feeling of security. Nevertheless, Zettlr currently auto-saves in every situation (so the shortcut is merely for the mental well-being). But the undo-history should only be reset in case you close and re-open the document.
How do I save my zettlr notes so that they are available on all my computers?
It works well to keep Zettlr notes synced across multiple computers and smart phone by locating the zettelkasten files in a cloud storage folder. NextCloud, DropBox, and OneDrive can all do the job.