In theory, Fascists supported the state as a means of mediating the dispute between labor and capital, meaning that the state would play an important role, but would not own industry per see.

Communists supports a system in which the state, as the representative of the working class, own the means of production. Which means the government would directly own all the industries in the countries.

In practice, this means that industries in Fascism are owned by individuals/shareholders, but would be controlled by the state to various degrees. And those individuals are capable of pushing back against the party-state with enough connections/money. Under Communism, the entire industrial sector is outright owned by the government and private ownership of (non-state approved) capital is banned.

Both systems are theoretically against western capitalism. In practice, Fascism (at least in Nazi Germany) ended up becoming an alliance of the state and capital, while under Communism, a new class of owners within the government were created.

if so then why are they on opposite sides of the spectrum?

Because the left-right political spectrum is over-simplified and not very accurate way of portraying political ideologies.

This question would be well suited for /r/debatefascism

Fascism doesn't fit on a left-right spectrum because it takes elements from both the left and the right.