If we take both Brahman and Īśvara, both of them are only seers and do not act by themselves. Both have Māyā; in Brahman as upādhi and in Īśvara as viśeṣaṇa. Upādhi can be separated from Brahman as it is external in nature and only a superimposition. But viśeṣaṇa cannot be separated from Īśvara as it forms integral part of Īśvara. Like Brahman and Īśvara who are differentiated with reference to Māyā, Kūṭastha (multitude) and jīva are differentiated with reference to avidyā. Avidyā (innate spiritual ignorance) acts as an adjunct (upādhi) to Kūṭastha, whereas in the case of jīva, it acts as viśeṣaṇa in the case of jīva.
What is the difference between Brahman and Īśvara? Brahman is Saccidānanda (sat-cit-ānanda), as described in Upaniṣad-s; Tattvabodha says, “evam saccidānanda svarūpaṁ ātmānaṁ vijānīyāt”. Thus, one should know himself as the nature of existence-consciousness-bliss). Īśvara on the other hand is Omnipresent, Omnipotent and Omniscient. Brahman is the cause of the universe and beyond perception. Brahman is Nirguṇa Brahman and Īśvara is Saguṇa Brahman or Brahman with attributes. It rules the universe. In understandable terms, Brahman can be called Paramaśiva and Īśvara can be called as Śiva. Māyā is superimposed on Paramaśiva and in the case of Īśvara, Māyā is an integral part, the stage of Śiva-Śakti union.
What is the difference between Kūṭastha and jīva? Kūṭastha is the Soul, which is immovable amongst the countless body, mind combination. On the other hand, jīva is an individual and many in number like Kūṭastha. Both Kūṭastha and jīva have adjuncts or upādhi. We call this jīva as the Self. Kūṭastha is the supporting consciousness in all the jīva-s and therefore Kūṭastha is also many like multitude of jīva-s. Kūṭastha and jīva are identified on the basis of avidyā. In Kūṭastha, avidyā is upādhi and for jīva, avidyā is viśeṣaṇa. In all the four discussed above, Brahman is always present, but in the case of last three, either due to upādhi or viśeṣaṇa, Brahman’s true nature is veiled. Removing this veil is the path of Self-realization~~Spiritual Journey - 1 2nd part ~~