The radicals 部首 are not just Mandarin Chinese, they are for all Chinese characters. They are the bases of the characters.
Take, for example, the radical 木 tree/wood, a pictograph (not obvious in modern characters. The limbs and the roots on the trunk). Put two of them together, and you get 林 forest, three are 森 a thick forest. If you combine it with 目 eye, you get 相, to examine (like a carpenter examining a piece of wood). If you add 幺幺 strings and 白 a drum (not white here), you get 樂, music, harmony, happiness. Put three 隹 birds on the tree, you get 雧(集) gather. If you add the sound 風 feng, wind, you get 楓 maple; 章 chang/zhang stripes, decoration on a tree is 樟 chang/zhang camphor because of its bark. Add the sound 市shi / market and get 柿 shi, persimmon. And so forth. I am using the Mandarin pronunciations, but it has worked for all dialects for thousands of years.
Does that give you an idea?