They ARE put in categories or ‘sections’ by bushou, and that is how dictionaries are organized. The bushou is the header of the section, and means ‘section head or header’.
No (I’ve already clearly stated that it is not, by which I mean that that is not what ‘bushou’ means. In some cases it might coincidentally be the original part, but that is not important here). The etymon is the original part of the character, if the character has gone through such an evolutionary path. Some characters exist in one form (A), either with one element or multiple elements, and then later one or more other elements (B, C, etc.) are added. In such a character, A is the etymon.
Most books don’t talk about etymons because they don’t have a clue about real etymology. They take a very shallow and simplistic approach to deconstructing characters, trying to break each 2-part compound into one semantic and one phonetic. But this can be quite misleading.
Take 採. The etymon (original character) is the right half or part (proportionally the right 2/3, actually). This is the main and most important part of the character in my judgement, but there is no formal definition of ‘main part’. The left part was added later, and happens to be the bushou for this compound, which is nothing more than the part used for finding the character in a dictionary. In internal function this left part is a redundant ‘hand’ semantic (the top of the right part is also a hand, but in another form).
If someone doesn’t understand that the right half of 採 is etymonic (i.e. the original character, since the left ‘hand’ was only added later) and that the right half thus carries both the meaning, as a hui4yi4 associative compound, and the sound, then they are likely to commit errors when analyzing the larger compound. For example, you might read in one book that 採 ‘to pluck, gather, harvest’ is made up of one semantic, 扌 shou3, hand, vaguely indicating some manual action, and one phonetic, 采, cai3. That’s a very shallow and not entirely correct analysis – how could the right half not also be semantic, especially since it specifically shows a hand harvesting plant matter, and since it specifically means ‘pluck, gather, harvest’? The right half is a FAR better semantic than the left half, no? So it’s stupid to simply term the right half a ‘phonetic.’
I’m curious to see what your books do say about the term ‘radical’ or ‘bushou’. Can you quote carefully from one or two to show us? Be sure to tell us the title and author, too. Note that there are many badly written books out there by people who don’t have a clue about characters and are happy to just regurgitate fairy tales about why the characters are put together in a particular way. Some of these books are attractively packaged with color pictures or cartoons, and are successfully marketed. On the other hand, the etymologists (those who study the REAL origins of characters) generally keep to themselves and don’t publicize what they know. They are scholars, not marketers.
I recommend you read some of these if you want better info:
DeFrancis, John (1984). ‘‘The Chinese Language: Fact and Fantasy’’, University of Hawaii Press, Honolulu. ISBN 0-8248-1068-6.
Qiú Xīguī (裘錫圭, 2000). ‘‘Chinese Writing.’’ Translation of his 文字學概論 (1988 PRC edition is in simplified Chinese; 1993 Taiwan edition is in traditional Chinese) by the late Gilbert L. Mattos (Chairman, Dept. of Asian Studies, Seton Hall University) and Jerry Norman (Professor Emeritus, Asian Languages & Literature Dept., Univ. of Washington). Early China Special Monograph Series No. 4. Berkeley: The Society for the Study of Early China and the Institute of East Asian Studies, University of California, Berkeley. ISBN 1-55729-071-7.
Woon, Wee Lee (雲惟利, 1987). ‘‘Chinese Writing: Its Origin and Evolution (漢字的原始和演變)’’ Originally published by the University of East Asia, Macau (no ISBN); reprint by Joint Publishing, jpchk@jointpublishing.com.