halacha theory - How does the Gemara decide which authority to rule like?

Consider various statements in the Gemara such as:

והלכתא כוותיה דאביי ביע"ל קג"ם

The Halacha follows Abaye in Ya'al Kegam

הלכתא כרב נחמן בדיני

The Halacha follows Rav Nachman by monetary issues

Or ones in the Rishonim such as:

הלכה כבתראי

The halacha follows later authorities

כל היכא דפליגי רב אחא ורבינא הלכה כדברי המיקל

Wherever Rav Acha and Ravina argue, the halacha follows the more lenient opinion

Where do these principles (and others like them) come from? Seemingly they're completely arbitrary, which is very unlike Gemara to do. How were these conclusions reached?