PCB Routing of CANH and CANL
There's not a lot of current on a CAN bus. Just enough to create the dominant state differential voltage of about 2.5V. At steady state there'll be DC load on the bus of about 60Ω due to the two 120Ω termination resistors. 2.5/60 = 42mA, so allow about 50mA. During transition from recessive to dominant there might be slightly more than this due to the charging of the bus capacitance. Usually there is not significantly more because the L/C ratio of the cable or tracks keeps the rising current low.
It's both. The bus itself should have a characteristic impedance of 120Ω, then this should be matched at either end by a differential impedance of 120Ω - the termination resistor. Usually the characteristic impedance is dominated by the L and C per unit length of the transmission line, while the termination resistor is mostly pure resistance. As long as the termination resistor in ohms numerically matches the characteristic impedance of the bus in ohms, then there will be no reflections resulting in the best signal integrity. So when designing the trace, aim for a characteristic impedance of 120Ω between CANH and CANL. Edge coupled microstrip is fine, but remember the impedance in between CANH and CANL - ground is largely irrelevant in CAN. When selecting a cable, aim for a characteristic impedance of 120Ω between CANH and CANL. Again, ground is irrelevant so reference planes are not important.


Usually you don't have to be particularly careful with bus impedance until your bus gets above 10m or so, or your speed gets above 500kbps or so.