How do I convert the scientific notation to it's original number?

What I'd suggest is abandoning CSV format: it's very old, and as such it's pretty basic. A better idea might be to Output Excel data as JSON - Office Scripts | Microsoft Docs[^] as you have a greater range of outputs than just "number" and "String"

But ... the problem is that when you export Excel data to any file type, it uses the number format you selected for the cell or column.
So if your cells have no specific format set, they are "General" - which is floating point which will get truncated and converted to scientific notation.
If it's "Number" format with zero decimal places, then it will be displayed and saved as an "integer" value with the full number of digits.

It's not a CSV problem, it's a spreadsheet problem!

Well...
If cell with scientific notation holds only 1.00004E+11, then you've got a problem. Why? Because 1.00004E+11 means 100004000000.
If this cell holds 100004438998 value, but uses scientific notation, you're able to save its original value to csv file.

You haven't show us the code which is responsible for conversion. I guess that you are treating all as a text (for example by using cell.Value.ToString()). That's why you're getting wrong values!

In case, you still want to convert values stored in csv file, take a look at example and its output:

string line = @"1.00004E+11;100004438998;";

System.Globalization.CultureInfo ci = System.Globalization.CultureInfo.GetCultureInfo("en-US");
long[] values = line.Split(new string[]{";"}, StringSplitOptions.RemoveEmptyEntries)
	.Select(x=> long.Parse(x, System.Globalization.NumberStyles.AllowExponent | System.Globalization.NumberStyles.AllowDecimalPoint , ci))
	.ToArray();

Result:

100004000000 
100004438998

Conclusion: save original values (rather then text) to csv file and you'll get proper values!