200mA is the current limit on a VCC or GND pin. So a Nano with 2 VCC pins can support 400mA.
However one must honor the per port current limits for sinking & sourcing current, see the notes after the Table 30-1.
The other numbers: 5V, that's typical supply voltage, used in logic circuits for a loooong time. USB outputs 5V as well. (5V +/- 5%, so 4.75V to 5.25V)
180 ohm, if you had a red LED, its Vf is about 2.2V. So a 180 ohm current limit resistor will allow about 15mA:
(5V - 2.2V)/180 ohm = 15.5mA, nice safe amount for an IO pin and driving an LED brightly.
360mA: 18 LED at 20mA each = 360mA. 60mA more than the IO pins should be supporting. See the notes, which discuss 300mA total max.