I would use System.Numerics.BigInteger.
There is no real limit on number size, only limit is memory.
Can be used like any other whole number type.
here's a couple of good references to read further
https://www.getcoding.co.uk/c-net/variables/biginteger/
https://learn.microsoft.com/en-us/dotnet/api/system.numerics.biginteger?view=net-8.0