The International Mathematical Olympiad contains highly non-standard problems involving the geometry of circles, triangles and lines which usually draws upon quite a few theorems and facts not typically covered in any high school curriculum. I know there are books like Solving Problems in Geometry by Kim Hoo Hang and EuclideanGeometry in Mathematical Olympiads by Evan Chen where such results can be found.
But where are they getting these theorems from? Unfortunately they don't talk about their sources much and I wonder what they might be drawing upon especially since classical geometry courses are pretty much extinct even at university level now. This isn't so much a request for textbook recommendations as it is a request for possible sources of lesser taught theorems of classical geometry.
Any information is much appreciated.