Are restaurants closed in the afternoon in Italy?

Yes — most kitchens close around 2:30 or 3pm and don't reopen until 7 or 7:30. If you finish a museum at 4:30 hungry, a proper sit-down meal is not available and no amount of walking will find one. What is open: bars for coffee and a panino, bakeries, pizza al taglio counters, gelaterie, and from around 6 the aperitivo places. Plan around it rather than fighting it — have a real lunch at 1, snack at 5, dinner at 8:30, and you've fallen into the Italian day instead of pushing against it.