I have read this question:
The fundamental confusion many have about black holes is thinking that they are discrete "things" surrounded by horizons and other phenomena. But they are actually extended spacetime curvature structures (that imply the various phenomena). The singularity is not doing anything and is not responsible for the gravitational field, it is a consequence of the field.
As far as I understand, as per general relativity, spacetime curvature is caused by stress-energy (not mass). This answer is using a vacuum solution to describe black holes, and you can read in the comments to that question that there is no need for any matter (or mass) to be present inside the black hole, it is just a vacuum, but spacetime itself is curved, and the gravitational field itself has the energy needed for the curvature itself. This includes the singularity itself, which in this answer is described as being "off the metric", that is not part of our spacetime, hence, it cannot cause the curvature.
Now, if the interior of the black hole, is a vacuum (the model is a vacuum solution), meaning the collapsed star's gaseous matter is not there (as far as I understand it is in the singularity), and the singularity is not part of our spacetime, then neither can cause curvature.
Again, GR describes curvature as being caused by stress energy. If there is no matter, no mass, nothing with stress-energy inside the black hole, except the singularity, but the singularity is not part of our spacetime, then what causes the curvature?
There are suggestions in the comments, that the collapsing star's gaseous matter transforms into the energy of the gravitational field itself. But I do not understand how electrons and quarks can transform into gravitons.
Still, how can the gravitational field itself cause the curvature, or how can it sustain itself? Gravity sustains itself, curvature means stress-energy in the gravitational field, and this energy causes curvature?
$1$. If black holes are just an empty vacuum of space inside, then what causes the curvature?