What makes the quality is the exact composition of the plastic (additives, color pigments), the consistency and accuracy of the diameter along the filament and how dry it comes out of the bag.

Bad composition could be very bad for bed adhesion and make the filament unusable.

Bad diameter will cause mostly visual quality defects

Humidifier will cause bubbles and strings, can be fixed by drying the filament.

Back when I started I ended up with some very poop quality filament that I paid way over priced ! Nowadays I have the feeling the cheep stuff is pretty good after some drying. At least as far as PLA goes.