“Shuwa-Shuwa” is an illustrated book of Japanese onomatopoeia

It's just part of the language. Everything has onomatopoeia. It's not just Toriyama who does it. You will find it in manga all over even stuff before his time. You'll also hear things like "jiiiiii" for people staring in anime too.

In Argentina we use "cri cri" (that sounds like "kree kree"). It's suppose to be sound of crickets.

Another fun one regarding silence would be じっと見る（JITTO MIRU）which is the sound of someone staring at something.

I haven't heard that dad joke yet.

This is a good explanation of Japanese onomatopoeia https://www.tofugu.com/japanese/japanese-onomatopoeia/

Onomatopoeia are commonly equated to phonomimes but can also include phenomimes and psychomimes. Phonomimes are meant to imitate an actual sound - like "bark" for a Dog (wanwan in Japanese). Most Japanese onomatopoeic expressions come in the double form (the sound repeated twice). There are of course っと and り forms of onomatopoeia but the double form is probably the most common and among the most known ones.

Phenomimes are onomatopoeia that depict something closer to an action than a sound - for example "powering up" (can't recall this one sorry) or "to be spinning around" (guruguru).

Doubled base onomatopoeia are not unique to Japanese either. Think of "bam-bam", "pop-pop", "click-click", "chirp-chirp", "oink-oink" or a bit less strictly "rattattat". I believe the technical linguistic term for this is called reduplication.

You might be right but as someone who studied Japanese and lived in Japan for a long time (15 years) at some point you (or I) grok it and it fits. You (or I) internalize the Japanese sounds of things and start to get it.

It also helps to use the Japanese pronounciations and accents. Some examples:

Pokki (pronouced Poh (rhymes with "go") then a pause, then Ki (rhymes with "me") Poh Ki, is the sound of stick breaking, which is why those chocolate sticks are called "Pocky"

https://youtu.be/IQQKnwZT55o?t=76

fuwafuwa (shown in the article), is clearly a soft sound. Vs say "crack" (an english onomatopoeia sound).

"zarazara" (zah-rah-zah-rah) vs "sarasara" (sah-rah-sah-rah) note: you need to roll the R one roll and say the word fast. "zarazara" is rough like sandpaper, "sarasara" is smooth like a kitchen counter. The zzzzz vs the sssss feels pretty clear after so many years of practice. zzzzz (bees go buzzzz in English) is clearly a buzziers more vibrating sound than sssss (snakes go ssssss in English)

"putsu=putsu" (pronounced puts-puts), vs "pata-pata" vs "zaa-zaa" are different sounds for rain. The rain is falling "Zaa-Zaa" is much harder (imagine hard rain on a thin aluminum roof) than "pata-pata" light rain (just enough for it to make noise) or "puts-puts", a sprinkle.

I mean, that's a matter of perspective, isn't it? English words like "bang", "chirp" and "snap" are considered to be onomatopoeia, even though their pronunciation has very little objective resemblance to the sounds they refer to.

It's more a question of whether the word's sound evokes a particular feeling or impression in the listener.

To some extent this is dependent on your spoken language and culture, but it can be surprisingly universal. See e.g. https://en.wikipedia.org/wiki/Bouba/kiki_effect

They're indeed often said twice in a row, but not always.

It means the same as for other languages. The doubling is commonplace but not exclusively used for all onomatopoeia. Japanese uses doubling of words in other situations and has a special repetition character when the first part is written in kanji to simplify writing them.

What do you mean by no equivalent? To my ears asking for "shuwa shuwa orange juice" is just like asking for "fizzy orange juice".

Fizzy wizzy juice. Fuzzy wuzzy was a bear. Buzzing bumble bee.

The difference is that it’s seen as childish in English and people stop talking this way in late elementary school, aside from limited uses (like buzzing)

I was very glad to find it here (I sent you the link via email as well). Can't wait to receive my copy!