'Bag' is translated 鞄/カバン in other words バッグ. I think that 'バッグ' is ハンドバッグhand bag, ショルダーバッグ/shoulder bag and トートバッグ/totebag, etc. I don't know these words are used as the same in English and Japanese well. Perhaps they are similar. (somebody help me.)
And 'bag' is translated the word '袋/ふくろ' as well. I assume that the 'plastic bag' is a bag that is prepared at supermarchets or convinience stores. It is called 'レジ袋/れじぶくろ'. You can search for about 'レジ袋' in wiki. Also there are the words 'ビニール袋/びにーるぶくろ', 'ポリ袋/ぽりぶくろ'.
Though I'm not sure. If people in 'Kansai dialects' maybe say 'ナイロン袋/ないろんぶくろ'.
関西/Kansai dialects is Osaka and around Osaka. (This explanation is pretty rough. >_<)
A lot of it is katakana, which is Japan's other syllabary. The sounds are all the same, they're just written differently. The use of hiragana or katakana depends on the context. Loadwords from other languages, such as バケツ (baketsu, bucket) are often written in katakana, while native japanese words are written in hiragana.
Japanese here. That kanji is not an uncommon one. The common kanji list is not reliable, as there are many more characters that are quite commonly used that don't make the list (e.g. 萎、which means "to wither" is quite commonly used but isn't part of the common list).
To see how often each kanji might be used in common media, refer to: https://www.bunka.go.jp/seisaku/bunkashingikai/kokugo/nihongokyoiku_hyojun_wg/04/pdf/91934501_08.pdf.
I'm wondering if there is a difference between how you make the Japanese "k" sound and the English "k" sound. Maybe it varies more than English according to the vowel that follows? Or maybe they're just different? Or there are regional accents we're listening to? I don't hear a "k" at the beginning of this word. What I'm hearing sounds like an h, or maybe maybe it's like a German ch or a Hebrew h.
You're right that it's vague, but something to consider is that かばん does not only mean a pocketbook, but can also be a briefcase, a messenger bag, or any other kind of sturdy bag that keeps it shape. In my dialect of English, it's normal to call any of those things a "bag", but obviously context is needed to differentiate them from other kinds of bags.
か ka（ が ga is not in this word) can be used on its own as a question particle. It is placed at the end of a sentence to turn a statement into a question.
学生です - I am a student 学生ですか - Are you a student?
寒くないです - It is not cold 寒くないですか - Isn't it cold? (is it not cold?)