This word is neuter gender; it's an "ett" word. (ett äpple, äpplet, äpplen, äpplena); "ett" words ending in vowel have 'n' form as plural. No, "-na" isn't always the way to make a plural definite, even with other neuter gender words. Example: (ett bibliotek, biblioteket, bibliotek, biblioteken). You are correct that there are lots of other words ending in 'n' where only an 'a' is added to make the plural definite, Example: (suddgummi, suddgummit, suddgummin, suddgummina). And you are correct that many words do add the "-na". Example: (en balkong, balkongen, balkonger, balkongerna).
I'm finding with Swedish, it is just a case of learning which, one is which and applying it correctly. We have a few words in English that, to someone who is learning, would find difficult to understand.
Apple = Apples
Child = Children
Mouse = Mice
Woman = Women
Person = People
Sheep / Fish = Sheep / Fish
All of these words are plurals, and in some cases, the plural form changes the way the whole word is pronounced. There's not always a specific rule in place, and if there is, then there are always exceptions. It's just the way that it is.