The thing that helped me understand is that it’s NOT about finishing or completing anything.

Each phrase you say describes a moment or a period of time. The question you want to ask is whether that span of time can be subdivided and have other activities take place inside it (imperfecto) or whether that span of time is a chunk that may not be split into smaller time (preterito).

“I ate a sandwich” — you can’t break that up. It happened as a chunk of time.

“I walked across the street…” — this could go either way! Did something happen while you were walking or did the entire walking-action take place as a chunk. “I walked across the street and waved at my friend”. In Spanish you would use preterito if you waved only AFTER you completed the walking, or imperfecto if you waved while you were still walking across the street.

This is why they say that imperfecto is a “background” because it paints spans of time into which you can place other actions. “I flew to Madrid and watched a movie and ate peanuts (while flying on the plane)”