What causes the separation needed for an oblique shock to form on a wing? Oblique shock waves form because :

"An oblique shock wave is a shock wave that, unlike a normal shock, is inclined with respect to the incident upstream flow direction. It will occur when a supersonic flow encounters a corner that effectively turns the flow into itself and compresses." - Wikipedia.

(Over this whole question, I'm referring to the first shock in a lambda shock, just to make an example)

For the oblique shock to form, there has to be a normal shock first, I think. The normal shock makes a separation bubble which makes the flow turn, making an oblique shock. My question is, what happens to that normal shock when the oblique shock forms? Does it disappear? If so, why? This answer and its comments has some really good info, but (for me) it didn't explain this part.

Also, if the flow behind a normal shock is always subsonic, how would the oblique shock form? I thought flow had to be supersonic to form an oblique shock?

(I'm usually bad about doing prior research to questions, but this time I did a fair amount. If I missed any sources that explained this, let me know!)