What is the grammar structure of "have been"

I'll assume that you mean the structure of the tense in the main clause.

It's a past tense called the perfect comprising the auxiliary verb "have", followed by a past participle, which in your example is "been" (the past participle of the verb" be").

In a perfect tense construction, the auxiliary "have" is itself inflected for primary tense, either present tense "has", or preterite "had". Perfect constructions like yours thus have compound tense. Examples with present tense "has" are called present perfect, and those with preterite "had" are called past perfect.

The past perfect tense contrasts with the preterite past tense (a primary tense form), sometimes called the simple past tense, which is marked as past by inflection rather than by the auxiliary verb "have":

I have been back from the trip for several months now. [past perfect tense: aux "have" + past participle "been"]

I was back from the trip by July. [preterite tense: "was", an inflected form of "be"]