This is going to be specific to a low bar squat
There are typically 2 sides of the debate regarding the knees during squats
Providing a visual for point 3 using Aimee above. On the left video, her knees drift forward too much at the bottom of the movement which is causing a few inefficiencies:
On the right side, we are focusing on letting her knees initially travel forward as much as they naturally need to during the top 1/3 of the squat, but then, not letting them drift any further forward for the remaining 2/3 of the movement.
By doing this (right video) her weight is keeping more in the middle of her feet instead of shifting to her toes, the bar is remaining in a straighter line over the middle of her feet, her hips aren’t shooting back nor is her chest falling forward as much. The right side is not perfect, she goes slightly into her knees too much on reps 3 to 5, however, her squat looks more efficient relative to last week.
Back to the two debates at the top of this post, the knees can travel forward over the toes if they need to depending on the measurements of the person, however, we should still have some control of keeping them back. People with longer legs may need the knees to go over the toes by some amount, but people with shorter legs can probably keep the knees behind or in-line with their toes.