Generally when an officer stops a driver suspected of IMPAIRED driving, (DUI and DWI are USA based terms), the officer will conduct some roadside sobriety tests. These tests give the officer an indication based upon the results, if a driver is sufficiently impaired to support a demand for a breath sample. It also provides the officer with indicators that he/she can use in their testimony before a court. Then if the officer is so equipped they will make a demand for a roadside screening device. They will then have the driver provide a sample, (unless of course the driver refuses to provide a sample). If the reading is sufficiently high, that the officer has grounds, to proceed, they will then arrest the driver and transport them to the station.
Once at the station the driver will be turned over to the custody of a "breath tech". A demand will then be made for a breath sample, for the breathalyzer. (Even if the driver had refused to provide a road side sample another request will be made. This is so that when they go to court the officers can show that the suspect was given every opportunity to provide a sample. Depending upon the tech and the department policy they may make 2 or 3 demands, (each time they will advise the suspect that a refusal will result in charges of failing to provide a breath sample). Once either two suitable samples, (the suspect provides two samples which are at least 15 minutes apart from each other), to confirm that they are somewhat "close" in value otherwise a third sample may be tken to ensure there wasn't a misreading, (normally the readings are within 20 ml, (or .20) of each other, and will indicate not only the level of intoxication but also if the suspect was "coming down" or still on their "way up" Once suitable samples are obtained the suspect is then returned to the custody of the arresting officer for either release or lodging in a cell.
Generally the breathalyzer is considered to be more accurate and reliable then the roadside screening devices.
Of course the above information doesn't apply if the driver is stopped at a "ride" spot check because there will a breathalyzer onsite in the back of a van.