Why is French devenir an intransitive verb?

There is a responsive word in English: become. If it is really difficult to understand, you can refer to Become Understand.

For example, beauty: beauty. This is obviously a copulative verb similar to the be verb. We know that copula verbs belong to intransitive verbs.

The same is true of devenir in France. In your example, un ami de la chine is not the object of devenir, but a supplementary explanation of devenir's action: What does he want to be?

She is getting better and better. The more beautiful she becomes. Un ami and belle in the orange you gave are actually the same ingredients.

When learning a language, it is sometimes unnecessary to pursue grammar excessively. Sense of language is more important than grammar. I hope I can give you some tips on learning.