WARNING SPOILERS AHEAD!
In Avengers: Endgame Smart Hulk snapped his fingers in order to bring everyone back from the avengers decimation, the snap Thanos done in order to wipe out half of all life, but we didn't get to see what Smart Hulk saw, unlike with Thanos, he seen his adopted daughter Gamora and there was a deleted scene from where Tony seen his daughter grown up when he done the snap. So that brings us to the point in which who Smart Hulk seen.
A small bit of backstory. After Thanos used the Infinity Stones to wipe out half of all life, the remaining Avengers found out not long after Thanos used the stone again. So Captain America, Black Widow, The Iron Patriot, Rocket, Nebula, Bruce Banner and Captain Marvel pursued Thanos in order to retrieve the stones to reverse what he had done, but then found out that he destroyed the stones.
Around 5 years later, many of those that had survived the snap were able to use time travel, by using the Quantum Realm, to retrieve the stones from the past in order to save everyone in the present. The mission was a success, even though a few branch timelines had been created and also resulted in the death of Black Widow, but Smart Hulk didn't let her sacrifice go to waste.
He used the stones to snap everyone back. Then later while in battle with Thanos from the past, 2014 Thanos, Tony used the stones again in order to wipe Thanos and his army out of existence, which killed him as he was only human. In a deleted scene Tony seen his daughter all grown u, who was played by Katherine Langford, but the scene was removed as test audiences found it confusing. If Tony and Thanos both had a visit to the Soul Realm with their snaps, then it would be pretty obvious that Smart Hulk did as well.
There are a few theories that are going around that Bruce Banner was able to see Natasha(Black Widow), since Hulk tried to bring her back, but she was telling him that she can't come back but that everything will be okay, reassuring him that he done well.
The other theory going around is that Bruce Banner comes face-to-face with the Hulk for the first time and they are able to talk to each other, showing that they are in it together.
The last theory going around is that Bruce saw his original love interest, Betty Ross. But, there is no confirmation on any of these theories to date.
My personal opinion is that both Bruce Banner and The Hulk were in the Soul Realm side by side coming face to face with Natasha(Black Widow) as they both had a deep connection with her. She was the one that was sent to recruit Bruce Banner for the Avengers Initiative and their connection grew into a romantic one in Avengers: Age of Ultron. That story line wasn't explored any further afterwards as the Hulk had left Earth at that point in time. Once Bruce came back in Avengers: Infinity War, there still seemed to be a connection, an awkward one at first but still a connection.
The other part that supports this is the fact that she was the only one that was capable of Calming the Hulk down so he could relax and turn back into Bruce Banner. So Natasha had a strong connection to both Bruce and the Hulk.
I believe that when Bruce Banner and the Hulk walk towards Natasha, she assures them that she is okay and that they both done an amazing job at saving the Universe. She tells them that everything is going to be okay.