What I'm asking is why the definite (este/esta/estas) was used rather than the indefinite (isto).
Does 'isto' have a plural?
No, "isto" doesn't have a plural. It points to something indefinite. Despite of being correct to say "Isto são maçãs.", it's unusual. In this very case, "isto" would demonstrate an unspecified (amount of) thing(s) that doesn't look like, but are apples...
I don't manage to hear the difference between "mac,ãs" and "massas" with that voice.
It's Duolingo's mistake O = Open N = Nasal S = Stressed
Maçãs = Ma(O)-çãs(NS), it's an "Oxítona", last syllable stressed
Massas = Mas(OS)-sas(O) it's a "Paroxítona", one before last syllable stressed
When we want to make a vowel sounds nasal, we can use Tilde (~, called "Til") on the vowel (a, with o, e, or alone, AND o, with e.)
Melão, Melões, Rã, Pães, Pão
Tip: If the vowel with a til isn't stressed, normally you put another grapheme.
Órfãos, Órgãos, Acórdão.