Upanishads along with Bhagavad Gita form core of Hinduism. After highly evolved Sages and Rishis invoked first words of God on mother earth they were known is Vedas and were very voluminous documents. As Vedas dealt with different subjects and were four in number, Rig Veda, Sama Veda, Yajur Veda and Atharva Veda, it was difficult for even learned scholars to indulge in jnana wisdom contained in Vedas.
In order wisdom contained in Vedas reached both scholars and masses, some Sages and Rishis developed independent treatises on various topics covering field of spirituality not religion. All Upanishads primarily dealt with individual topics. They can be considered as further elaborations so as to deal with the subject matter in depth. All Upanishads are spiritual treatises and deal with important chapters of spirituality contained in Vedas.
Scholars interested in spirituality dealt with Upanishads while pundits indulging in religious practices directly indulged in Vedas.
Later on as Lord Krishna advent on mother earth, the essence of all Vedas and Upanishads necessary for human beings to reach state of enlightenment, self realization was encompassed into Bhagavad Gita. In present times anyone wanting to reach God in present life time, reach stage of self realization, god realization can indulge only in teachings contained in Bhagavad Gita. Indulgence in Vedas or Upanishads is not necessitated.
Upanishads encompass highly evolved spiritual doctrines and can be understood taking path of contemplation (chintan) only.
In my pursuit of God since six years of age I indulged both Bhagavad Gita and Upanishads but never Vedas, it was never necessitated. Indulgence in Upanishads simplified understanding doctrine of Bhagavad Gita in totality.
Every single Upanishad was compiled by a single Sage or Rishi. All in all above 200 Upanishads existed as of today of which 11 are primary.