Firstly, "your garage" is definite, i.e. if you add an indefinite noun to a pronoun, it becomes definite. Now, about the adjectives - I'll explain it by using a few examples. "A new garage" in Arabic would be "كَراج جَديد" (neither the noun, nor the adjective takes the definite article), "the new garage" would be "الكَراج الجَديد" (both, the noun and the adjective are preceded by the definite article), "the garage is new" would be "الكَراج جَديد" (only the noun takes the definite article). I hope, it's a little bit clearer now. If you still have any questions, feel free to ask.
The complete ending sounds of the sentence above (according to the standard grammar) is:
كراجُكِ جديدٌ يا ساميةُ.
"karaajuki jadiidun yaa samiyatu."
When "Harf nida" (tool/letter for calling) "يا" met "mufrad 3alaam" (a name composed from a single word) like "سامية", it has the Damma sound, "u". "saamiyatu". Please see this link for further explanation https://arabicblog.info/harf-nida-in-arabic/
However, on daily basis, Arabs spell "saamiya", ie. by omitting the ending sound.
Conclusion: For "saamiyatan", it is grammatically incorrect, is an audio glitch. And, we can assume that "samiatou" which you have heard is also an error because Duolingo try to avoid ending sounds.
JARIK means "Your (to a female) neighbor (a male)" while JARAK means "Your (to a male) neighbor (a male)".
So, we use JARIK in this sentence because "Your" (Samia) is female and her neighbor is male. "Your neighbor" (ie. Samia's neighbor)
If "Your" refers to Bob, then the sentence will be JARAK - "Your neighbor" (ie. Bob's neighbor).
Note: Both JARAK and JARIK above (ie. in the nominative case) are a Slang/Dialect and not Standard Arabic.
karaajik كراجِك is a Slang/Dialect which Arabs never spell the second ك sound (or -kah in your transcription).
However, if we follow the Standard Grammar, the last ك is pronounced. So, it will be like this: كراجُكِ "karaajuki" (the ك is spelled as "-ki" which means to a female and "-ka" to male. Hence, it should be "-ki" (and not "-ka") because Samia is female.
Conclusion: the -kah sound is an audio glitch. :))