From Ephesians by Peter S. Williamson, commenting on Ephesians 4:9-10:
What does “he ascended” mean except that he also descended . . . ? What is Paul’s logic? Paul is interpreting Ps 68:19, which addresses God, saying, “You went up.” He makes the logical inference that to speak of God ascending implies that he had previously descended, since God’s dwelling is in heaven, above everything else. The Old Testament sometimes describes God’s intervention in human affairs as his coming down or descending (e.g., Gen 18:21; Exod 3:8).
Paul understands Ps 68 to be speaking of Christ. After all, it begins, “Let God arise, let his enemies be scattered” (RSV). When did God arise and scatter his enemies? At the resurrection of the Messiah, of course! And Paul tells us to where the Messiah descended: he went down into the lower [regions] of the earth. There are various interpretations of this descent. Most likely it refers either to Christ’s incarnation, when he emptied himself of heavenly glory (Phil 2:7) and came down among us (John 3:13), or to his burial in the earth. Alternatively, it could refer to Christ’s descent into the realms of the dead upon his death, where he preached “to the spirits in prison” (1 Pet 3:18–22).
In any case, the one who descended is the same person who has now ascended far above all the heavens, namely, Christ. As in Phil 2:6–11, Paul marvels that the one who came so far down has now been raised so high up. God had a purpose for this: that he might fill all things. Here, as in 1:23, “fill” means to exercise divine authority everywhere (echoing Jer 23:24) so that the Messiah might be Lord over all.