Lefty is correct, the advantage of open source hardware is the inherent diversity and dialog of the projects.
As for Aref, connecting this to VCC is all well and good but having access to it is invaluable if you are running from the 5v of usb but using a 3v sensor and need to calibrate it. Another one of those features similar to the extra ADC pins - they can be used when needed or ignored otherwise.
As to the CC-BY-SA license this does not apply to the name Arduino (that would be the trademark) but instead the design of the board itself. So if you make a derivative work of the Arduino hardware, as the Pico and I believe your om328p is, these works should adhere to the terms described in the CC liscense found here.
Cheers,
Brian