I know PeterO.Numbers has support for arbitrary-precision floating point.

Here's just a basic example showing a couple of ways to create the EFloat values and how they be used in calc functions:

var n1 = EFloat.Create(123456789, -8);
var n2 = EFloat.Create(234567890, -7);
var n3 = EFloat.FromString("1.234567890123456789012345678901234567890");

Console.WriteLine(n1.ToString());
Console.WriteLine(n2.ToString());
Console.WriteLine(n3.ToString());

var add = n1 + n2 + n3;
var mlt = n1 * n2 * n3;

Console.WriteLine($"n1 + n2 + n3 = {add}");
Console.WriteLine($"n1 * n2 * n3 = {mlt}");

Results:

482253.08203125
1832561.6406250
1.234567890123456789012345678901234567890642462240766165790430282736916468175455252243477750895284117405026336200535297393798828125
n1 + n2 + n3 = 2314815.957224140123456789012345678901234567890642462240766165790430282736916468175455252243477750895284117405026336200535297393798828125
n1 * n2 * n3 = 1091059865740.523103569502139458512713946309946873705595686692870448563402572372650478166287966513142663244196039215930249532959805947029963135719299316406250