It turned out that Xiao Hei had appeared in this plaza early on, right when he entered the second cave and was being transported.
At the time, there were still eight teleportation gates remaining on the plaza.
Furthermore, this dark passage had also opened up at that time.
People returned intermittently, entering the dark passage and not coming out again.
Xiao Hei stayed because he was waiting for Shi Hao; as for the big yellow dog, that was purely out of fear of death and instinct for survival—not at all about loyalty.
Xiao Hei said that the teleportation gates vanished one by one, and Shi Hao realized that this coincided with the time they had picked the Divine Lotus.
In other words, after the Earthcore Fire Lotus was harvested, this place provided an additional option for teleportation: to the outside world. Additionally, another dark passage opened up because of this.