Formula(s) to combine columns in to 3rd column

I've seen several examples of something close to what I'm wanting to do, but I've not seen this specific example discussed yet.

I have 2 columns with data and blanks and I want to combine the data in to a single column, but there's a caveat, some of the cells in each column both have data.

Column B - Employee ID

Column C - Employee Email

Column D - Where I want to combine

If employee ID (column B) is blank, populate column D with Email from column C

If Employee ID is present, populate Column D with the numbers in the "000000000"format

If column B and C both have data, use column B (employee ID) in column D

I've tried all forms of CONCAT, TEXT, If, OR, etc.