How to work with WS2812B 16x16 matrix panel from aliexpress
It's just a string of WS2812B LEDs that have been &quot;bent&quot; into a grid. You treat it exactly like any other WS2812B strip of LEDs.
You have 256 LEDs, and as far as the Arduino is concerned, they are just in a line numbered 0-255.
The &quot;grid&quot; arrangement is solely down to you to you to calculate.  Fortunately 256, or 16x16, is very easy to work with: you have a byte which can represent any one of the LEDs, and each half of the byte (a nibble) is one coordinate.
The traditional calculation to convert X and Y into a linear number is:
num = x + y * width

where width is the number of LEDs in the X direction (16).  However since we're working with a width of 16 that can be optimized to use bit-shifting and bitwise operations:
num = x | (y &lt;&lt; 4)

That is assuming LED 0 is on the left (either top or bottom) and LED 1 is one place to the right.
Will you need an external power supply for it? Yes, most certainly (and they even provide you with a dedicated power connection for it). 256 LEDs, if all are turned on full brightness at once, could consume up to about 15 amps (20mA per channel, three channels per LED, 256 LEDs).