Actually, "Ore" is a rude/pompous way of referring to oneself, though it can be used in a joking way. More or less, it's like saying "my amazing self" instead of "me." Also, "boku" is masculine, but sounds kind of kiddish, which is why men tend to eventually shift from using it to saying "watashi" (polite neutral) or ore. Also, girls can use "boku" too. If she's more masculine/rough-and-tumble, she may be more likely to use "boku." Singers use it often in music, too, because it tends to fit more easily into lyrics, having 2 syllables. "Atashi" goes along with the over-the-top pink frills and lace aesthetic; it's super girly and also a bit kiddish. There are others, too; these are just the most common/well-known ones.
Japanese always uses kanji where possible. You will never see "watashi" written this way. Many of the words that DuoLingo teaches have kanji, but Duo spells them in hiragana because they think some kanji wouldn't be very useful. I wish that Duo would teach more kanji, especially for words as basic as "I".
Well thats probably because this is a complete beginners section of the course and litetally each section titled hiragana. Because of the complicity of this language its more efficient to learn hiragana katakana first and then advance up to kanji, if you are already familair with both of the beginner alphebets then i believe it does give you an option to test out of them.
"Watashi" is usually written using kanji... but that isnt all. The phonetic spelling is actually "ha-ta-shi" spelled like "はたし" Even though it is pronounced "wa" the hiragana for "ha" is used. There are many words that are pronouned wa but spelled ha because there was no わ katakana before. Watashi (はたし, I) Watashitachi ( はたしたち, we) and even konnichiwa (こんいちは, hello) are all written with ha instead of wa. Teenagers today sometimes spell these words with wa instead of ha, but it is kind of like using slang and incorrect.
konnichiwa (こんいちは, hello) are all written with ha
this is written with は because it is a lexical phrase, shorthand for the sentence: 今日はご機嫌いかがですか？ (konnichi wa go kikeni kagi desu ka) (this reading of 今日is borrowed from chinese and meaning in english is closer to 'this day' or 'now a days' instead of 'today')
most literally that sentence says: "as for this day, spirits/health in what way [is your] state of being?" less awkwardly versioned as something like, "how are you this day?"
that's a lot to say every time you meet someone in the daytime, so over a long time it eventually got shortened into the modern japanese greeting "今日は” (usually styled: こんにちは ) "as for this day..."
こんばんは has a similar origin, substituting "this evening" for "this day"
Ore (wa) and Boku are the write choices if you are men. Watashi is more generic all the situation type of word. If you want to give yourself something unique in your style of speaking you can use the above words or even wagahai which is archaic word mostly known by the poem Wagahai wa Neko de Aru by Natsume Souseki, and you can speak about yourself in third person. This is all informal, stick to formal in work related situations.
It's not rude. It was rude 20 years ago; today most japanese young men of a certain age (old enough to not want to use 僕 young enough to not want to use 私) use 俺。How they characterize it in writing/sns varies a little bit, I've seen おれ、オレ、and 俺。Yes, you shouldn't use it to your boss or other formal setting, but it's pretty commonly used informally today.
try to watch more anime or look around Japanese people, maybe you can rarely hear them saying 私 for self pronouncing, it's usually for females and some elder males (for we, they use 我々), some of the others use one these common pronounces instead: 私 (わたくし ver.) あたし (modified ver. of 私) used by younger women, I use this when I speak Japanese 俺 (おれ) Frequently used by men. Establishes a sense of masculinity. 俺 (おら ver.) 僕 (ぼく) can be use by males or females in most of the songs you hear, mostly used by males of all ages, but very often used by boys own name (for example うまる) used by small children and younger women, considered cute and childish for details, check this website https://en.m.wikipedia.org/wiki/Japanese_pronouns
私/わたし/Watashi is for if you are a girl or in a formal situation using this in a casual setting as a man would make you sound fem. Also "I" can also be 僕/ぼく/Boku which to my understanding is used by young males or if you want to sound youthful. A more manily way of saying "I" would be おれ/Ore but this is a fair warning this isn't a very polite way of saying "I" unless you are with close friends and or you're just a macho man.
私 (わたし) is just the word for "I" or "me". 私は is how you'll see it when the speaker is the topic (a grammatical point we'll get to in a bit) of a sentence.
watashi wa watashi desu.
I am me.
watashi wa gakusei desu.
I am a student.
If it's obvious from context, then you can omit the subject.
I am a student.
niku wa tabemasen.
I don't eat meat.
They are not the exact same, hiragana and katakana is the phonetic "alphabet" that could be used to phonetically spell words out, and kanji is the shortened (and chinese origin) symbolic "alphabet". For example, "わたし" is watashi, meaning I. But "私" is also said as watashi and also means I.
Pronouns are rarely used in general and only necessary when clarification is needed. So instead of saying "I" they wouldn't mention it at all.
In which case, many English phrases get longer than the Japanese
And a phrase testmoogle on here likes to use as an example
木漏れ日 komorebi - “The sunlight that filters through the leaves of a tree.”, Why is the English so long!?