Mmm, I thought that particular estar rule was for physical locations of people/objects? A habit it is neither and can't be defined by physical location. You can say "it's a habit here in this town". But you can't say "I'm just taking my habit over there, it's over here at the moment". "Here is a habit of mine" is however common where I live. But this is more of an idiomatic expression, "here is" actually meaning "I'm about to tell you".
It is common in these sentences for "es" to mean "it is" if there isn't a subject but of course what makes this sentence tricky is there seems to be a subject, "aquí". It doesn't help that we've been trained to translate sentences into English that don't make any sense, which is why I felt comfortable writing a sentence where I was presenting someone with a habit. I guess what makes the sentence wrong is "aquí" can't be a verb in Spanish, only a an adverb.
Aquí can't be a noun, I think you mean.
Nevertheless, you can still use "Aquí está algo" to mean "Here is something" - the important thing to note is that literally it's actually more like "Something is here" ("algo" is the subject, it's just in a different position). In fact I believe it's more common to say "Aquí hay algo" ("There is something here").
You can't apply the same reasoning to "ser" because that expresses equivalence (to put it simply). "Aquí es algo" amounts to "Here, it is something" or "It is something here", as you see in Duo's sentence. Alternatively, if you decide that "algo" is the subject (like it was with está), you end up with "Here, something is" (i.e. it exists), which is an unusual sentence.
It is a sentence I would say. here is an example.
"Where can I see some customs of your country?" "Here is a custom." while pointing to a custom that someone can visually see. If the custom does not need to be explicitly pointed out, then it is certainly a valid sentence (though not often used, it would still be better than "Here it's a custom" where it has no explicit frame of reference).
I believe "Here is an apple" would most commonly be expressed "Aquí hay una manzana", or literally "Here there is an apple". That's because, in Spanish, "aquí" can't be a noun, it's an adverb, which means it can't be the subject of the sentence. Instead a subject is provided by "hay" with its implicit "there".
"Aquí está una manzana" is also valid, however you have to be careful that you don't misunderstand the literal meaning. Once again since "aquí" can't be the subject, in this case "una manzana" is actually the subject, even though it comes after the verb, and the literal translation is something like "An apple is here" or "Here an apple is". In fact you might as well say "Una manzana está aquí", since it's exactly the same (apart from maybe a minor shift in emphasis). That's a subtly different construction to the English "Here is an apple" (which uses "here" as a noun and the subject of the sentence), even though both mean the same thing in practice.