I2C: How does the Master recognize an ACK from a specific slave?
The master does not know which slave pulled the SDA low to send the ACK. It just means that at least some chip responds to the address sent. In reality there could be accidentally or intentionally many slaves at the same address so they all would pull the SDA low to send an ACK bit.
However the assumption is that each chip has one (or multiple) unique address so if you send an address, only one chip will pull SDA low for ACK.
If no chip pulls low, then that means nothing on the bus responds to that address, i.e. a chip might be missing, faulty, disconnected from bus, or have just been configured to wrong address.