How do I code using a cell?

no I'm trying to make a conversion calculator in two cells. I have all the numbers and letters already set but I can't figure out how to make the equation read each letter/number in the cell and convert it

You cannot do this in an equation.
It requires a custom function to do this as loops are required to pull out the numbers from cell to lookup the associated letter add it to a string, skip the comma (or delimiter) and then move onto the next number in the cell. Repeat until there are no more numbers. Display the string in the cell.

It will help if you posted the cell ranges of your letter/number cells, or even a screenshot or example spreadsheet

@Thank you for your fine explanation.

Here is a function that does what you need.

In a module put the following code:

Function ConvertCode(code As Range) As String

Start = 1
While Start < Len(code)

    Pos = InStr(Start, code, ",", vbTextCompare)
    If Pos = 0 Then Pos = Len(code) + 1
    StrValue = Mid(code, Start, Pos - Start)
    Start = Pos + 1
    Set rng1 = Range("A1:A31").Find(StrValue, , xlValues, xlWhole)
    StrValue = rng1.Offset(0, 1)
    ConvertCode = ConvertCode & StrValue
    
Wend

End Function

This assumes the following (as I do not know the layout you are using):
1.Column A contains the number.
2.Column B contains the character

The Range contains 31 rows as the list of characters includes other non-alphabetic characters such as Space, #, $, as these may be useful. Add other characters as you need but adjust the range accordingly.

To use the function, lets say the code you want to convert is in cell D2, in another cell, put:

=ConvertCode(D2)