Anything but typical

中文也有相同的表達方式:
(1) 他除了掙錢, 甚麼也懂.
(2) 他除了掙錢, 無麼也不懂.

(1) 除了壞事, 甚麼也做.
(2) 除了壞事, 一概不做.

Referring to your questions:

(1) anything but typical = can be anything, except 'typical'.
Your understanding is correct.

(2) nothing but typical = 除了是 typical 外, 一切都不是.

(3) has to read the complete sentence and judge individually.

(1) The boy is anything but typical.
Does it mean that the boy is not typical? YES

(2) The boy is nothing but typical
Is this sentence correct? If so, what does it mean?
It means that he is typical only but nothing else.

(3) How to interpret the adjective after "but"?
But means something is different from or not the same as what you have said.

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