In John 3:16 starting with ἠγάπησεν (aorist), the verbs shift to tenses signifying past actions. Does this mean Jesus' discourse with Nicodemus stopped with 3:15, and 3:16 starts John's narrative?
Lang gives three basic positions:
The dialog stopped and John picked up the discussion in v16; position of Erasmus
The dialog continued at v16; position of Lange
The middle ground; Robertson explains it as John summarized the rest or the dialog; also Goodspeed.
Through Erasmus (see Lücke, Ι. p. 543) the view has become current with later scholars, Kuinoel, Paulus, Tholuck, Olshausen, Maier, and others, that from ver. 16 the Evangelist continues the discussion on his own part. The disappearance of dialogue, the preterites ἠγάπησεν, ἦν, the term μονογενής peculiar to John, and the general character of the discourse, are taken to show this. But this hypothesis has been with good reason contradicted by Meyer [p. 168], Stier, Baumgarten-Crusius, and myself in the Leben Jesu ΙΙ. p. 508. John’s coloring is in fact admitted elsewhere; why not here? Lücke proposes a middle view. -- Lange, J. P., & Schaff, P. (2008). A commentary on the Holy Scriptures: John (p. 133). Bellingham, WA: Logos Bible Software.
...while not a quotation from Jesus is a just and marvellous interpretation of the mission and message of our Lord. In verses 16 to 21 John recapitulates in summary fashion the teaching of Jesus to Nicodemus. -- Robertson, A. T. (1933). Word Pictures in the New Testament (John 3:16). Nashville, TN: Broadman Press.
3:16. Whether this verse was spoken by John or Jesus, it is God’s Word and is an important summary of the gospel. -- Blum, E. A. (1985). John. In J. F. Walvoord & R. B. Zuck (Eds.), The Bible Knowledge Commentary: An Exposition of the Scriptures (Vol. 2, p. 282). Wheaton, IL: Victor Books.
John 3:16–21. These verses are the evangelist’s meditative comment upon the discourse of Jesus just recorded. The theme of this paragraph has been described as “the motive and effect of divine revelation in the Son” (Burton). What was the motive, and what the effect, according to these verses? -- Goodspeed, E. J. (1917). The Gospel of John (p. 10). Chicago, IL: University of Chicago Press.
Verse 15 seems to be transitional, moving from Jesus’ words to the evangelist’s commentary, as here we find Johannine language: “that whoever believes in him may have eternal life” (cf. 3:16: “that whoever believes in him should … have eternal life”). Many red-letter editions of our Bibles show verses 16–21 in red, indicating they are Jesus’ words; I believe, however, that these verses should not be put in red letters as they are almost certainly the evangelist’s commentary. This would in no way detract from the incredible declaration made in the beloved verse John 3:16: “For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life.” -- Köstenberger, A. J. (2021). Signs of the Messiah: An Introduction to John’s Gospel (p. 54). Bellingham, WA: Lexham Press.