I notice that when Chester County dispatches EMS to a life threat incident, they dispatch an ambulance, a medic, and a QRS. My question is are the medics on board the ambulance, or is the QRS a team that boards the ambulance, or is each thing in a separate vehicle?
It depends on which EMS squad is dispatched and where in the county the call is.
There are only a handful of EMS squads which run MICU ambulances (EMT and medic on board).
MICU units - 25 (Longwood), 67/167 (Phoenixville), and 87-6 (Uwchlan's rig at Station 6 between 0700-1900).
In most parts of the county, the medic responds in a separate vehicle from the BLS ambulance.
ALS Chase Cars - 2 (Berwyn), 4 (Malvern), 46 (Downingtown), 87 (Uwchlan, save for MICU 87-6 example above), 91 (Chester Co Hospital), 93 (Brandywine Hospital), 94 (Southern CC EMS).
A few areas have Quick Response Service (QRS) units, which can either be a separate chase car, or in some cases a fire engine. These units are occasionally referred to as Squad xx or QRS xx.
QRS providers in Chester County - 26 (Atlgen - fairly new, they usually respond in Engine 26-3), 27 (Cochranville), 31 (Sadsburyville), 38 (Thorndale), 41/43 (Coatesville FD/typically respond when 41 EMS is committed on a call elsewhere), 48 (Glenmoore), 49 (East Brandywine), 58 (West Chester University), 61 (Kimberton), 73 (Ludwig's Corner), 74 (Upper Uwchlan).
Some BLS companies with a single ambulance will respond in a TAC or Squad unit when that rig is out of service (such as 5 - East Whiteland or 44 - Westwood).
A typical dispatch for BLS, ALS, and QRS:
Ambulance 89 (Elverson), Medic 87 (Uwchlan), and QRS 48 (Glenmoore) dispatched to an ALS call in Wallace Township. The call is in 48's first due fire territory, which is closer to the 48 house than 89, in Elverson Borough (near the Berks County border). Even if 89's bus gets out a couple minutes before Tac 48, responding as the QRS, Tac 48 is closer and is capable of providing BLS care until the ambulance arrives.
That explains Chester County EMS in a nutshell.