請問產品Made in Taiwan是否為The products are made in Taiwan的簡寫,或是made過去分詞當形容詞adj用, 加介詞in, 再加名詞Taiwan,構成片語用,謝謝,?

It is the state or quality of being passive; no adj + prep. in.
The passive voice in grammar is the form in italic, type in the sentence:-
eg:-The products (were made) in Taiwan.
ie  the verb phrase containing "be + pp past participle"=were made=verb phrase.
Re:-The products, which were made in Taiwan, were expensive.
When relative pronoun is used accusatively in defining clause=adj cl., you can say "adjective clause".Never say "adjective + prep in " in your question.

🔷Made in Taiwan：[ph.] 臺灣製造
例1. to be made in Japan
例2a. This was made in Britain.
例2b. My camera was made in China.
例3a. These goods are of Japanese manufacture.
◽例3b. These goods are {of(prep.) Japanese(adj.) make(n.)}. 華語譯：這些商品(是){出自日本的品牌}。
例3c. These goods are of Japanese made.
例3d. These goods are made in Japan.