Theophylact of Ohrid

After their preaching, the apostles gather to Jesus. This should be a lesson for us that we too, having been chosen for some ministry, should not depart from obedience to the one who chose us or exalt ourselves before him, but should acknowledge him as head, turn to him, and report to him everything we have done and taught (one must not only teach but also act).

Commentary on Mark ↗

Augustine of Hippo

This is said to have taken place, after the passion of John, therefore what is first related took place last, for it was by these events that Herod was moved to say, This is John the Baptist, whom I beheaded.

Catena Aurea by Aquinas ↗

Bede

And the apostles, gathering together, reported to Jesus all that they had done and taught. Not only did the apostles report to the Lord what they had done and taught themselves, but also what John suffered while they were engaged in teaching, or his own disciples or those disciples of John reported to him, as Matthew describes. Hence follows that which comes next:

On the Gospel of Mark ↗

Glossa Ordinaria

The Evangelist, after relating the death of John, gives an account of those things which Christ did with His disciples after the death of John, saying, And the Apostles gathered themselves together unto Jesus, and told him all things, both what they had done, and what they had taught.

Catena Aurea by Aquinas ↗