Last n digits of BigInteger

I'm looking for fast way to get n last digits of number, represented as BigInteger, lets say, m.

Calculating m mod(10^n) (or converting to string) seems too expensive, because the sizes of n are pretty big. Is there faster way ?

Can you use BigInteger.DivRem() or BigInteger.Remainder()?

For example, to get the last 4 digits, "DivRem" by 10000:

BigInteger b1 = new BigInteger(123456789);
BigInteger b2 = new BigInteger(10000);

BigInteger.DivRem(b1, b2, out var b3);

Console.WriteLine(b3); // Prints "6789"

Or to get the last 20 digits of a much bigger number:

BigInteger b1 = BigInteger.Parse("1234567890123456789012345678901234567890");
BigInteger b2 = BigInteger.Pow(10, 20);

var b3 = BigInteger.Remainder(b1, b2);

Console.WriteLine(b3); // Prints "12345678901234567890"

Not sure if this is any faster than using .ModPow() though...

Addendum: I did try a benchmark:

[MemoryDiagnoser]
public class Benchmarks
{
    [Benchmark]
    public void DivRem()
    {
        BigInteger.DivRem(b1, b2, out _);
    }

    [Benchmark]
    public void Remainder()
    {
        BigInteger.Remainder(b1, b2);
    }

    [Benchmark]
    public void ModPow()
    {
        BigInteger.ModPow(b1, 1, b2);
    }

    static readonly BigInteger b1 = BigInteger.Parse("12345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890");
    static readonly BigInteger b2 = BigInteger.Pow(10, 100);
}

With these results:

|    Method |      Mean |    Error |   StdDev |   Gen0 | Allocated |
|---------- |----------:|---------:|---------:|-------:|----------:|
|    DivRem |  96.11 ns | 0.968 ns | 0.905 ns | 0.0076 |     120 B |
| Remainder |  86.89 ns | 0.476 ns | 0.445 ns | 0.0045 |      72 B |
|    ModPow | 167.48 ns | 0.726 ns | 0.679 ns | 0.0045 |      72 B |

So it appears that DivRem() and Remainder() is almost twice as fast, at least for the numbers in the benchmark. Of course, you should try it with the sort of numbers you're actually using because the results may differ!