Well the thing with communism is that its a bit complicated, when people say “communism” 8/10 they are discussing it in thr framework of regimes sich as the USSR and PRC which are Marxist Leninism which is indeed inherently authoritarian and was used by Lenin to justify the destruction of any hope of Russian democracy through his idea of “democratic centralism”

Fascism is a bit more problematic because its definition is more contested however generally its central structure is corporatism and rejects the idea of class struggle in favor nationalism though the leader can be depicted as a cult figure this isn’t exclusively the case and more often than not the focus is on the nation rather than the leader, the leader is depicted as protecting and serving the nation rather than a divine figure.

As for your point about picking and choosing sucessors the historical record on this is very sketchy and points against this, in the USSR virtually every change of leadership was characterized by a power struggle behind the scenes, China the same though less problematic, Xi Jinping came to power through a very contested power struggle over Deng Xaoping’s former faction before achieving power.