In the 70s, the population groups: "Taiwanese" descendants of Han who moved across the Taiwan straits a few hundred years ago and mostly displaced the natives. Their native language was a dialect of Chinese that was not mutually comprehensible with Mandarin... (1/3)
...but most also knew Mandarin (the official dialect, from northern China) "Mainlanders" people who came over after WW2 and the Communist takeover of the mainland. They mostly spoke Mandarin, sometimes with strong regional accents (2/3)
"Aborigines" remnants of the original inhabitants. Ethnic Asian but not ethnic Chinese. (3/3)
I’d assumed you had meant aboriginal Taiwanese, but sounds like that’s not the case?
Back then, "Aborigine" was not "Taiwanese". But I'm way out of date on current usage. FWIW, Taiwanese were maybe 80% of the population. If you wanted to see an Aborigine you mostly needed to go to a mountain scenic spot / tourist trap, which I never did.