Your question seems to indicate that you're not aware of the difference between a numeric value and its representation.
Consider this:
int number = 100; // representations:
string numberAsBinary = Convert.ToString(number, 2); // = "1100100"
string numberAsOctal = Convert.ToString(number, 8); // = "144"
string numberAsDecimal = Convert.ToString(number, 10); // = "100"
string numberAsHexadecimal = Convert.ToString(number, 16); // = "64"
These are four different representations of the same value.
Mathematical operations work with values, not with representations of values.
So the answer to your question is: You don't even bother about the representation of your values while applying mathematical operations to them. The only points where you care about the representation are when you want to assign a value to a variable from a string which represents a number and when you want to output the value of a variable in a certain representation.
Here's a link where you can find a method to convert a string with a binary representation of a number to a BigInteger:
C# Convert large binary string to decimal system - Stack Overflow[
^]
The reverse, formatting a BigInteger to a string with binary representation I'll leave for you as an exercise :)