Updating data in a cell with data from another cell when a specific outcome is reached

I am trying to make it so that when one cell (B1) reaches a value of 0, A1 which is text entered by user, is now replaced with the data in C1 which is also data entered by the user. Meaning that formulas cannot be placed in either A1 or C1 as they are data inputted by user (B1 already has formula calculating its value.)

Instinctly I want to use =IF(B1=0,A1=C1,"") but obviously that doesn't work as A1 = C1 is checking if they equal each other, not replacing A1 with C1.

There's probably going to be a simple solution to this but I cannot figure out a way to make it work.