A survey's fields are organized into steps. Multi-step surveys walk the visitor through one step at a time (with a progress indicator and Next/Back buttons); each step holds an ordered list of fields you add via the step editor.
Use Add step to append a new one — the editor automatically scrolls to it. Each step has two admin-facing fields (a name and a comment) that help you keep track of what the step is for; these aren't shown to survey visitors, only in the editor. Removing a step removes its fields with it.
There's no separate "one-step survey" mode or toggle — it's simply a survey whose steps list has a single entry. Nothing about the editor or the published widget behaves differently; a one-step survey just never shows the multi-step progress indicator or a Next button, since there's nothing to advance through.
The first numeric-scale field (rating, matrix, or slider) you add anywhere in the survey is automatically flagged as the main evaluation question, and the first text field is automatically flagged as the main feedback question — unless you've already flagged one yourself. That's what later shows up as "general satisfaction" and the "default feedback text" on the Responses page, so it's worth adding your primary rating/comment field first if you care which one gets picked automatically. You can always change the flag afterward in a field's own Meta settings.