Baptism and Transfiguration
In contrast with Matthew, Mark, and Luke, John omits both the baptism and transfiguration:
16 When He had been baptized, Jesus came up immediately from the water; and behold, the heavens were opened to Him, and He saw the Spirit of God descending like a dove and alighting upon Him. 17 And suddenly a voice came from heaven, saying, “This is My beloved Son, in whom I am well pleased.” (Matthew 3 NKJV. Also Mark 1:10-11, Luke 3:21-22)
1 Now after six days Jesus took Peter, James, and John his brother, led them up on a high mountain by themselves; 2 and He was transfigured before them. His face shone like the sun, and His clothes became as white as the light. 3 And behold, Moses and Elijah appeared to them, talking with Him. 4 Then Peter answered and said to Jesus, “Lord, it is good for us to be here; if You wish, let us make here three tabernacles: one for You, one for Moses, and one for Elijah.” 5 While he was still speaking, behold, a bright cloud overshadowed them; and suddenly a voice came out of the cloud, saying, “This is My beloved Son, in whom I am well pleased. Hear Him!” (Matthew 17. Also Mark 9:2-7, Luke 9:28-35)
At both a voice from heaven announces This is My beloved Son.... The voice is never explicitly identified as God or the Father, but that is a reasonable conclusion.
Psalm 2:7
After His death and resurrection, Jesus is preached as the Son of God, sometimes using Psalm 2:7:
“I will declare the decree:
The LORD has said to Me,
‘You are My Son,
Today I have begotten You. (Psalm 2:7)
God has fulfilled this for us their children, in that He has raised up Jesus. As it is also written in the second Psalm: ‘You are My Son, Today I have begotten You.’ (Acts 13:33)
For to which of the angels did He ever say: “You are My Son,
Today I have begotten You”? And again: “I will be to Him a Father,
And He shall be to Me a Son”? (Hebrews 1:5)
So also Christ did not glorify Himself to become High Priest, but it was He who said to Him: “You are My Son,
Today I have begotten You.” (Hebrews 5:5)
When Matthew, Mark, and Luke are read with the the phrase today I have begotten you in mind, there may be a logical question - Does "today" refer to the baptism or the transfiguration? In other words, since the voice from heaven at the transfiguration and baptism said, This is my beloved Son... one might attempt to connect either event with "the day" Psalm 2:7 speaks of.
Conclusion
John begins the Gospel with the Prologue which proclaims the pre-existence and the unique monogenesis Son. Then throughout the Gospel, it is Jesus who proclaims His unique relationship with the Father. It is Jesus who states plainly He is the beloved Son. For example:
16 For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life. 17 For God did not send His Son into the world to condemn the world, but that the world through Him might be saved. 18 “He who believes in Him is not condemned; but he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God.
(John 3)
In place of the voice from heaven at the baptism, the Fourth Gospel gives the Baptist's words (which he had received from above):
Behold the Lamb of God who takes away the sin of the world (John 1:29)
In place of the voice from heaven at the transfiguration, John gives the voice from heaven near the time of glorification:
27 “Now My soul is troubled, and what shall I say? ‘Father, save Me from this hour’? But for this purpose I came to this hour. 28 Father, glorify Your name.” Then a voice came from heaven, saying, “I have both glorified it and will glorify it again.” 29 Therefore the people who stood by and heard it said that it had thundered. Others said, “An angel has spoken to Him.” 30 Jesus answered and said, “This voice did not come because of Me, but for your sake. 31 Now is the judgment of this world; now the ruler of this world will be cast out. 32 And I, if I am lifted up from the earth, will draw all peoples to Myself.” 33 This He said, signifying by what death He would die. (John 12)
By omitting the transfiguration the Fourth Gospel gives greater focus on Jesus as the pre-existent Son, and His future radiance, His glory, which is His death and resurrection.