> How do the 46 characters map into the 90 characters?
Because there are actually more than 46 characters.
> Do they map the same way for both hiragana and katakana?
Yes. That's also how we do conversion between hiragana and katakana. By adding/subtracting 0x60.
Hence, that's very much fortunate that someone wrote about it.
Perhaps most interesting in this respect is the comparison between the Devanagri ि and the Thai ใ which both appear before the consonant that they’re attached to, but in Thai the input will be ใ + ค to get ใค (so you input in the order of appearance rather than the order of pronunciation) while in Devanagari, the input would be क + ि to get कि (so you input in pronunciation order rather than graphic order).
Worth noting that «-a» is often implied and both, Thai and Devanagari (and in nearly all other Brahmi and Pali derived scripts), and is implicitely derived by the language speaker, and therefore is dropped from the spelling most of the time except for specific cases.
...
That said, culturally Japan seems like exactly the kind of place where, were they English-speakers, all the kids would absolutely be required to learn perfect cursive.
In practice, there are no less than 4 separate scripts that are used in Japanese: hiragana, katakana, kanji, and romaji, and some mix of all 4 can appear in the same sentence.
It's not so much analogous to cursive, which is a different "style" of writing the same "thing" – katakana and hiragana developed at different times for different groups and came to play different roles, and there are (usually) semantic implications to which are used.
But from what I understand Hirigana is used more for Japanese words, and Katakana more for loan words from other languages.
It actually leads to a nice shortcut for some words. If I’m reading Hirigana I’ll try to match that with words in Japanese that I know. However, if the word I’m looking at is Katakana, I’ll flip that off and start trying to match phonetically.
I assume with fluency this all becomes automatic, but I’m a ways off from that yet!
Katakana, hiragana, and kanji are all in active use--that's why they don't fall away.
Kanji are your primary word base. They are sort of like root words in English.
Hiragana often serves as kind of a marker--endings of certain words as well as phrase markers (particles). These are particularly important because Japanese does not normally break words with spaces.
Katakana often denotes a foreign phonetic word or foreign names. Login is particularly good example for this forum: ログイン (ro-gu-i-n).
Japanese speakers actively use the differences as cues when reading. Watch a native Japanese speaker try to puzzle out Japanese learning materials for non-native speakers. If everything is written in hiragana (not uncommon for beginning materials), native speakers often have to puzzle over things a bit before they work out what a sentence says. This is one of the reasons why you want to get to Kanji as fast as possible when learning Japanese--the differences in script are important for reading comprehension.
You can see all of these in play on the Asahi Shimbun webpage: https://www.asahi.com/
Actually when you look at the use of English in Japanese media, you’ll quickly notice a lot of unnatural-looking overuse of uppercase. That’s because to them it feels natural to use uppercase the same way they use katakana.
As between the versions for the same mora, they get sorted with tertiary differences as: hiragana comes before katakana, small comes before regular-size, and for katakana regular width comes before halfwidth. There's also a "circled" set of the katakana that sort after the halfwidth ones.
So they're equivalent (or not) depending on how you're doing the collation/comparison.