I don't actually think it's all that obvious from the picture just what you have; and if I had to guess, I say something chrome plated, not solder over copper, unless it was soldered within the last week. Too uniform and shiny to my eye. Pull the escutcheon out and take another picture to provide more data for guessing...and try a magnet if you think it might be steel or iron, but my money is on threaded chromed brass, pretty typical for bathroom fixtures. If pulling the escutcheon out continues the same color (allowing for dirt) all the way to the wall, my confidence increases that that's what you have. If it changes color drastically then copper looks more likely.
As for it not unthreading - well, that happens. There's limited access behind a toilet, so you can't swing a big wrench, it's wet quite a lot of the time from condensation in the summer (most climates) plus any leakage, and it doesn't get moved for decades at a time. Harsh cleaners can add to the corrosion potential. Determination, penetrating oil and heat can all help, but sometimes all that happens is the pipe snaps off. The Joy of Plumbing.
If it was a typical soldered copper pipe and you actually tried to unthread it with the force typically needed to unthread a threaded connection that has sat for a few years, the pipe would have twisted flat, in my experience. So sorting out what it really is probably beats just taking a wrench to it. The area very near the wall is a fine place to scrape, scratch or even lightly file to help sort out what it is, if it's not obvious from just pulling out the cover - you won't be making a compression connection that far back.