It depends a little on the application, OBD2 gateway modules always have one built in and expect the diagnostic device to have the other,
In passenger buses, its not uncommon for them to have 2 physical resistors at the end of the bus, with stub plugs all along the wiring for things to connect in. so if a device is hooked up that has a resistor built in and cannot be turned off, its generally connected to an end plug with the physical resistor removed in its place.
And in trucks, its generally, a physical resistor inside the canbus gateway module (they are becoming quite common, low speed bus + high speed bus + kline of to weird peripherals) and a plug mounted resistor at the end of the line.
Most 250Kbps buses can handle up to 3 termination resistors (40 ohm bus) without issue, but some modules transceivers can act intermittently. Mercedes OEM gear is really sensitive to this.
Older J1708 buses just relied on every node having a 10K resistance between A and B, and no more than about 32 nodes to a run.
The resistors main purpose is to normalize the differential voltage to 0 when the bus is idle, A lot more transceivers are center biased these days, e.g. pulled to a 2.5V rail generated in the chip, rather than some bias divider per wire, but its still not perfect, take a given transceiver, and you may find 40mV difference across them at idle with no resistance,