This is called tanwin or nunation. Long story short, it is a type of vowels in Arabic (and I don't know why Duolingo is not putting the marks here).
Tanwin can come in 3 flavors: -un, -an and -in depending on the status of the word (nominative, accusative, ...etc) when the word is indefinite. Thus, you can somehow think of it as being a marker for the indefinite noun (like a/an in English somehow) - though this is just a short explanation.
The sign for -un Tanwin is:
or maybe some books (specially for children) would write it as:
Thus, the vowel you've heard before smart is actually the end of the adjective before it (amríkiyatun) - American (f.adj.).
Originally, in Arabic, Bint is (daughter of), and used in names (some countries still use this system), like X bint Y meaning X daughter of Y. There is some orthographic story here but I don't want to complicate it.
On the other hand, ابنة (ibnah) is daughter (the feminine of ابن ibn, son).
On the other hand, girl in Arabic is فتاة (fatáh) or (fatát). And there are a number of words in classical Arabic to note a girl, but the basic word is this one.
Here on Duolingo, they are using a dialect mixed with the standard or classical, making some sort of "traveler's Arabic" - so they are using "bint" here for daughter and girl.