If you mean the capitalization, the rules are the same as in English for the word "one" - if you start a sentence with it, capitalize it, otherwise leave it in lowercase. So effectively capitalization doesn't change the meaning. Since it's not a sentence but an expression, you can eithe capitalize it or just leave as одне місто.
From Duo's point of view, it doesn't matter either as Duo ignores capitalization and punctuation ayway.
They mean the same, no difference in meaning, if used as "and". You might choose one and not the other depending on the phonetic environment, there are even rules prescribing when to prefer і, when to prefer й and when to prefer та, but the native speakers don't really follow them, so, from the practical point of view, treat them as interchangeable to not overcomplicate the matter.