Let us make it clear. “Veda” has come to mean anything what its people want that word to mean – from Chāndasa mantras to even Bible in the modern times.
If you are asking about verses of sages, verses of sage composers the “mantradraṣṭāraḥ”, then that is composed of Ṛks and mantras in chandas – that includes complete Rigveda and Sāmaveda with a huge chunk of Yajurveda and Atharvaveda saṃhitās. Traditionally, for the mīmāṃsā usage esp., Veda term meant both mantra and brāhmaṇas.
Brāhmaṇas are prose works which are mythologies/narrations centered on a ritual theme, and explaining the verses used there in the specific context. They are clearly later than mantras as they presuppose the existence of mantras, and they are not divinely inspired.
Upaniṣads are the latest layer of texts in Vedic Sanskrit (by that, I refer to authentic śākhā-preserved Upaniṣads and not the ones like Allopaniṣad or Rahasyopaniṣads). They are teachings and speculations, prayers and benedictions collected in the final layer of texts in Vedic Sanskrit, mostly attached as chapters of Āraṇyakas or Brāhmaṇas and in one case (Īśopaniṣad) a part of saṃhitā itself.
Mīmāṃsakas believed that yajña was beneficent because it was the best karma – the “ritual action” with the “right verse” yielded the fruit. Therefore, their pramāṇas were saṃhitās + brāhmaṇas (as without brāhmaṇas, mantras would no longer be restricted to ritualistic realm, which goes against the agenda of mīmāṃsakas).
Vedāntins on the other hand, based their religion on various speculations and teachings in Upaniṣads, deeming them to be confident and coherent. They never looked beyond Upaniṣads and dismissed off the whole mantras as mere “karmakāṇḍa” against which the Upaniṣadic verses became “jñāna kāṇḍa”.
Upaniṣads, Āraṇyakas, Saṃhitās and Brāhmaṇas are the texts in Vedic Sanskrit which are transmitted orally in a śākhā of Vedas (a school/branch of Vedic recension). This is what is meant by when one says Īśopaniṣad for instance belongs to Śukla Yajurveda. (It belongs to Śukla Yajurveda following school)