Current flow in ground plane of non isolated dc dc converter
You should first study how buck converter works.







simulate this circuit &ndash; Schematic created using CircuitLab

The input current (green) flows through input capacitor and it it is close to DC, depending on capacitor's value and properties, in your case about 4.2A.

The switch current (red) is pulsed. It's duty cycle depends on converter's duty cycle - about 12/48 = 25% in your case. It's magnitude is close to the output current with some additive depending on inductor's value. So in your case the switch current is pulsed current with a magnitude of about (16-20)A at 25% duty cycle. 

The diode current (blue) is also pulsed and it is complementary to the switch current - it has (almost) the same magnitude (16-20)A and it's duty cycle is the rest - in your case (100%-25%)=75%.

The output current (yellow) depending on output capacitor's value and load consumption is close to 16A DC.

You can assume that the plane between C2(-) and D1(A) carries nearly DC current, of course depending on inductor's value. The current between D1(A) and C1(-) is highly pulsed. All other currents are close to DC. 

In the perfect PCB layout C1(-), D1(A) and C2(-) are placed as near as possible to each other to minimize pulsed current paths.