Since your updated information is not needed to explain, let me answer without it. (I am sorry to say this, the update only gives me further writings with sorry.)
To me, it seems to transfer a adverb [有名] to a noun, is that so? Could someone help to explain, thx.
有名, mostly and commonly as 有名な as the na-adjective. (But as I commented Mr./Ms.'s comment line it can be used as a noun too. Ex. 有名税,
explained in the comment, 有名校【ゆうめいこう】, meaning "a famous (highly regarded) school")
Actually, I would like to explain 有名な as the na-adjective only here. Personally, I would like to use the word as noun 有名 + the auxiliary
だ. Since many say 彼【かれ】は有名だ, meaning he is famous.
Now, 有名な is here the na-adjective. Since the na-adjective conjugates, in you speech, 有名な is taking the 連体形. The pattern of the conjugation of the na-adjective of the 有名な is as the followings.
命令形(Imperative) (basically non-existing)
++Please refer to here for the Japanese conjugation forms in English.
Now, 連体形, after which comes a noun, and since the postpositional particle
の can be swapped by 物（もの）, meaning a thing (noun) and categorized as 格助詞.
To me, it seems to transfer a adverb [有名] to a noun, is that so?
The answer is NO. Firstly 有名 is not an adverb. Now as for
Could someone help to explain
From the above forms of the conjugation of the na-adjective of 有名, your speech becomes as the following.
最も (adverb, meaning "the most" in English)
有名な (na adjective of 有名), taking 連体形
の (particle, like substantive, meaning can be a substance)
が (particle, showing the previous part speech 最も有名なの is a subject)
ウーロン茶 (noun, Oolong tea)
で (auxiliary), 連用形 (suggesting another speech will continue after)
So, の is the particle explained above.
The whole translation (including your update).
The most famous Chinese tea in Japan is Oolong.
Have a nice day.