As far as the Trinity and God’s simplicity, the distinction is that there are three “whos” but only one what. What the Father is is the same as what the Son is is the the same as what the Holy Spirit is. I know what can have a broader meaning in English, but in by this “what” I am referring to the essence or substance of God. There is only one simple substance. There is a Trinity of “persons” as this one simple substance subsists as three relations. The distinction is only relational. There are no parts or separate beings. St. Thomas Aquinas, following Saint Augustine of Hippo, thought if it in the following (but grossly simplified) way:.
God knows himself and is known by himself. Knowing and being known are distinct.
God loves and is loved by himself. Loving and being loved are distinct.
It is God’s simple substance, that he knows and loves from and as his essence, that leads us to be able to understand these relationships as being distinct hypostases or persons, whereas even though I may know myself and love myself I am only one person of many parts.
It takes some reading and meditating on.