What’s the difference of watts at different resistance (ohms)?

I’m new to car audio and I’m trying to find out if 500 watts at 4 ohms would produce the same sound levels as 500 watts at 2 ohms. Or would the same sound levels be produced if it was 500 watts at 4 ohms or around 900-1000 watts at 2 ohms? Also, do rms speaker ratings pertain to 4 ohms or any resistance?
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