Are transitive verb phrases intransitive verbs?
I think I’m asking that probably because some transitive verb phrases goes before prepositions.
Verb phrases can be both transitive and intransitive, in just the same way as single-word verbs.
He digs up the road.
"digs up" is a transitive verb phrase, needing an object.
He cheers up.
"cheers up" is an intransitive verb phrase.
Note that both "dig" and "cheer" are verbs that can be either intransitive or transitive depending on meaning.
So a transitive or intransitive verb phrase may contain a transitive verb or an intransitive verb. There is not a fixed relationship between them.