I'm aware of two types of recirculation pumps.
The traditional system runs a connection from the most distant hot water tap (often the kitchen or master bath) back to the hot water heater. From there, you would have a check valve and a pump that moves the hot water from the tank, to that distant fixture, back through the return line that was added, and into the hot water tank via the cold inlet. With a home run PEX system, you don't have a most distant fixture.
A second type avoids the need to run the return line to the hot water tank by connecting the distant fixture by using the cold water line for the return. This means your cold line would end up being filled with water that had gone through the hot water tank, which is a significant drawback to this system (you really don't want to drink the water that's been through a hot water tank with all the sediment).
Since PEX is typically a home run install, this means you'll need to install a device on each fixture you want to have recirculated. If, for example, you only hook up the bath tub, you won't see any benefit at the sink if they are each a separate home run. Each of these connections would be independently controlled, either with a thermostat, timer, or some other on demand switch, so that your cold water lines don't fill with hot water. You will also need to have a power source at each fixture to run the pump.