The Trinity is a mystery, so you will never fully grasp the mystery. Even the seraphim and cherubim don’t have a full grasp of the mystery.
I’ll pray that the Holy Spirit gives you faith, understanding, and knowledge regarding the mystery of the Holy Trinity.
Here is a story about what happened to St. Augustine when he tried to understand the mystery:
The seraphim and the cherubim are part of visions and are symbols for the future glorified saints. The symbolic/spiritual uses are far too in depth to speak about here and would take a verse by verse exposition of Isaiah, Ezekiel and Revelations as well as the few other parts in which they are mentioned. If you wish, I can give a crash course in the subject. Suffice it to say, they are symbolic visions of the future saints surrounding the throne.
Also, the mystery thing is a cop out. It is saying “I do not understand so I am not meant to understand.” Really, it is “my belief contradicts Scripture, so I will say that it is a paradoxical mystery.”
“And he said, Unto you it is given to know the mysteries of the kingdom of God: but to others in parables; that seeing they might not see, and hearing they might not understand.” Luke 8:10
All mysteries have been revealed in Christ.
“their hearts may be encouraged as they are knit together in love, to have all the riches of assured understanding and the knowledge of God’s mystery, of Christ, in whom are hid all the treasures of wisdom and knowledge. I say this in order that no one may delude you with beguiling speech.” Col 2:2-4
So either you understand or you do not understand. Those who do not understand are in the like with the Pharisees.
“God sends upon them a strong delusion, to make them believe what is false, so that all may be condemned who did not believe the truth but had pleasure in unrighteousness.” 2 Thess 2:11-12
It is our duty to seek the Truth, and we shall find it in Christ.
“And I say unto you, Ask, and it shall be given you; seek, and ye shall find; knock, and it shall be opened unto you. For every one that asketh receiveth; and he that seeketh findeth; and to him that knocketh it shall be opened.” Luke 11:9-10