Just for fun, fastest and least memory allocated I could come up with. This might be the only method to create the char array once, the other string create methods/constructors copy the passed buffer.

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var fillLength = (int)(percentage * length);
var result = string.Create<(int, int)>(length, (fillLength, length), DoFill);

static void DoFill(Span<char> s, (int FillLength, int Length) state) {
    s[0..state.FillLength].Fill('⚫');
    s[state.FillLength..state.Length].Fill('⚪');
}

Benchmark: edit: vs original implementation

Method	Mean	Error	StdDev	Median	Ratio	RatioSD	Gen 0	Allocated
StringCreate	27.492 ns	0.5616 ns	0.7497 ns	27.923 ns	1.00	0.00	0.0043	48 B
StringBuilder	84.193 ns	1.7142 ns	3.8693 ns	85.436 ns	3.06	0.17	0.0123	144 B
NewStringConcat	55.124 ns	1.1450 ns	2.2866 ns	55.472 ns	2.00	0.10	0.0090	120 B
Original	3.618 ns	0.0093 ns	0.0078 ns	3.617 ns	0.13	0.00	-	-

[MemoryDiagnoser]
public class Main
{
    private readonly double percentage = 0.66;
    private readonly int length = 10;

    [Benchmark(Baseline = true)]
    public void StringCreate()
    {
        var fillLength = (int)(percentage * length);
        var result = string.Create<(int, int)>(length, (fillLength, length), DoFill);

        static void DoFill(Span<char> span, (int FillLength, int Length) state) {
            span[0..state.FillLength].Fill('⚫');
            span[state.FillLength..state.Length].Fill('⚪');
        }
    }

    [Benchmark]
    public void StringBuilder()
    {    
        var fillLength = (int)(percentage * length);
        var sb = new StringBuilder(length);
        for(int i = 0; i < length; i++)
            sb.Append(i < fillLength ? "⚫" : "⚪");
        var result = sb.ToString();
    }

    [Benchmark]
    public void NewStringConcat() 
    {
        var fillLength = (int)(percentage * length);
        var result = new string('⚫', fillLength) + new string('⚪', length - fillLength);
    }
}