Not the theoretical or philosophical differences, but what differences exist in society/government between the two?
Fascism and socialism are both totalitarian ideologies, but they are different both in theory and practice.
Three major distinctions that have a bearing on actual practice between the two ideologies are:
1) Marxism is a globalist ideology and Marxist regimes act accordingly. One of the first priorities following the Bolshevik Revolution was to establish the Communist Internationale and foment revolution abroad. Fascism, on the other hand, is nationalistic, and fascist regimes act accordingly. It’s true that Mussolini’s Italy sought to establish imperial possessions in Africa, but it did not have global ambitions and its imperial ambition is not inherently a fascist agendum. Neither fascist Spain nor Portugal was similarly acquisitive.
2) Under Marxism all of the means of production are owned and operated by the state. Under fascism the means of production are largely owned by private concerns and are strictly regulated by the state. Both are planned economies, but the ownership distinction is non-trivial.
3) Marxism is officially atheist and under Marxism religion is either banned entirely or severely persecuted. Under fascism religion is tolerated or even encouraged if it acknowledges the secular authority of the state.
These distinctions are not trivial.
I’m sure you noticed that I revised “socialism” to “Marxism” in making my distinctions. That’s because US progressives have a tendency to include several small, moderately successful, Nordic welfare-state market economies in their ill-informed definition of socialism.