Nothing opens. The phrase has stuck because it sounds like it describes a sensation, and I have watched it leave people working hard on the wrong thing for years.
Three different reasons you are stuck
The first is muscular length, which is what everyone assumes. Tissue that has not been taken to a long position in a while and objects when it is. This one does respond to patient work.
The second is neural. Your nervous system will not permit a range it does not believe you can control, and it is right to be cautious. This one responds to strength at the end of range, not to stretching, which is why some people stretch for a decade and go nowhere.
The third is bone. The angle of your femur and the depth and orientation of your hip socket vary enormously between people, and no amount of anything changes them. Some bodies will never sit in a full lotus and the reason is architectural.
Why this matters more than it sounds
Because the three want opposite treatments, and because the third one has no treatment at all.
Someone with a bony limit who keeps forcing is not making progress towards anything. They are grinding a joint, and the eventual outcome is a hip problem in their fifties caused entirely by trying to reach a shape that was never available to them.
How to tell the difference
Roughly: if the end feels springy and warm and eases over minutes, that is tissue. If it feels like a wall that arrived suddenly, often with a hard quality and often at a very consistent point, that is likely bone.
I am far better at this distinction than you are, because I can see you, and it is one of the best uses of an hour with me on your own.
The useful reframe
Stop asking how deep the position is. Ask whether you can control it, breathe in it, and get out of it under your own power.
That is trainable for everybody, unlike the shape, and it is the thing that actually transfers to being able to use your hips.