
I mean, it this the Titan? Is the cave system part of its organism? Is that what is meant by "we're inside it?"
No bloody idea , but if lyrium veins are part of the beast , and there doesn't seem to be a clear definition of where its body start and when it ends ...well if there's lyrium veins you're walking on it , or inside it.
I'm not sure the cave , plants and stuff are really the Titans.
I mean we have all sort of things in our bodies like bacteria etc , they're inside us but not really part of us.
Edit :
I'm watching some youtube video ,trying to clear up the confusion.
So here's what's said in game.
Shaper Lady reading an old rune about Titans :
"It shapes the stone .It is the stone.It sculpts the world within and without"
Inquisidumb :If Titans "shape the stone" could they have actually created it?
Shaper Stupid :The Stone must have existed first.
Inquisidumber :Then The titans would be the very first children of the Stone....
Later after several rocks and lyrium blast to the face :
Inquisinotgettingsmarter : "You said you are a child of the Stone, not a child of the Titans."
Shaper Who Has Lost It: I'm not certain what I am , but the Titan recognised me .Like a parent hearing its child's voice.
Maybe I'm the stupid one but I guess the rune saying "IT IS the stone" should have settled the whole business. 
Then the Titans would be being made of the stone and they could shape it like one shapes muscles or I don't know change a haircut.