A phrase is never a part of speech, only a word.
"Central idea" is a noun phrase. It refers to the main point or topic of a piece of writing, speech, artwork, etc.
"Central" functions as an adjective, describing the type of idea. "Idea" is a noun, which is the main subject of the phrase. Together, "central idea" forms a noun phrase.
The term "part of speech" is a noun phrase, which is any word or group of words based on a noun or pronoun (without a verb) that can function in a sentence as a subject, object of a verb or a preposition. A noun phrase can be one word or many words. The word "part" is a noun, "of" is a preposition, and "speech" is a noun. object of the preposition.
The term "main idea" is a noun phrase. It refers to the primary point or central theme of a text, paragraph, or discussion.
Prepositional phrase
The phrase "in addition" is a prepositional phrase in which "in" is the preposition and "addition" is its object. This phrase, as a phrase, is not a part of speech, although it may function as one, probably an adjective or adverb.
A prepositional phrase.
On the mountain top is a preposition phrase, not a part of speech.
The term "part of speech" is a noun phrase, which is any word or group of words based on a noun or pronoun (without a verb) that can function in a sentence as a subject, object of a verb or a preposition. A noun phrase can be one word or many words. The word "part" is a noun, "of" is a preposition, and "speech" is a noun. object of the preposition.
That I am is a phrase, the individual words in the phrase are parts of speech. That -- demonstrative, determiner I -- pronoun am -- be verb
adverb phrase
verb