Hebrews is different from most of the other New Testament letters in that there is no formal introduction. There are no greetings nor indications of who the letter is addressed to. In the opening section the writer quickly establishes what the letter is primarily about, namely the supremacy of Christ.
In the Old Testament there are all sorts of writings: history, prophecy, poetry, etc; and it covers a vast time scale. In all this God was speaking to His people. Now He has spoken through his very own Son. There is a different order of magnitude in the nature of the revelation given in Christ and all that went before. Christ is the heir of all things, and through Him all things were created (note that Paul says the same things in Colossians).
The prophets and other godly people in the Old Testament represented something of the character of God, but all were flawed. Jesus is the exact representation of God. When we see Jesus we see exactly what God is like. Moses reflected the glory of God, but Jesus is the radiance of God’s glory.
Jesus provided purification for sins and then sat down at the right hand of God. His work was completed. So He became as majestic as His name was majestic. There was a tendency in those days to venerate angels, hence the references to the supremacy of Christ over the angels. Indeed, false teachers today will sometimes talk a lot about angels. We should always beware of someone who talks more about what they have “seen” rather than about Christ Himself.