Generally speaking, drug conspiracy cases work in the same way as other criminal investigations where an officer gets a lead and finds out where a drug distribution might be taking place or someone is in possession with intent to distribute. Then the officer will likely investigate that individual to find out who their co-conspirators are with respect to the furtherance of that drug-related offense.
For example, the officer might look at the cell phone of an arrested person to see who have they been communicating with to find leads or information on who else might have been involved in the furtherance of that drug-related crime.