Macao is a Special Administrative Region on the Pearl River Delta: a Portuguese-painted historic centre, a reclaimed Cotai Strip of casino-resorts, and Coloane village still serving minchi and egg tarts that are not a Hong Kong copy. You can walk from a 16th-century church ruin to a bus that smells of fried pork chop buns in twenty minutes. The place is small and vertically loud.
The pataca (MOP) is official; the Hong Kong dollar is taken almost everywhere, often near one-to-one. Spend leftover pataca before you board the ferry — they are not useful on the Hong Kong side. Plugs are Type G, the same square-pin British style as Hong Kong, at 230V. Ferries from Sheung Wan or Kowloon still make the crossing in about an hour; the Hong Kong–Zhuhai–Macao Bridge is the other way in if you are already on a bus tour.
Cantonese is the street language; Portuguese survives on street tiles and in some offices; Mandarin and English work in casinos and hotels. 999 is the emergency number. Eat at a dai pai dong–style place in the old city, ride a bus to Hac Sa, and treat the strip as optional architecture, not the whole trip.
A first visit that works is the historic peninsula plus Coloane, with Cotai only if you want that particular spectacle. Do not try to "do Macao" as a four-hour ferry dash from Hong Kong unless you only want the ruins of St. Paul's and a tart. The better version sleeps one night and hears the city after the day-trippers leave.