I'll zoom right into this line from your question:
Everyone says that deep squats will destroy your knees...
"Everyone" in this case is patently incorrect and repeating gym nonsense. According to the prevailing logic of gym goers we should all curl and bench press, maybe doing the leg press or smith machine when we notice our legs look pencils. Then back to curling in the mirror.
Mark Rippetoe (author, trainer) has written extensively about the benefit of full depth squats for your knees. From Rippetoe's writing:
The muscles on the front of the thigh are the quadriceps. They attach
below the knee to the “tibial tuberosity” – the bump at the top of the
shin bone – just below the kneecap. When they pull this bone forward,
the knee extends and the force at the tendon attachment is directed
forward relative to the joint. In contrast, the hamstrings pull
backwards on either side of the knee at their attachments, which
balances the forward force from the quads. This happens in a correct
squat when the hips move back and the torso leans forward. The balance
of forces is optimum at a position just below parallel, and protects
the joint so well that a correct squat can be safely performed even
without an ACL – a person like me.
You should go as deep as you can without rounding your back, and this will be deeper on a front squat because of the load placement. Your hamstring (bicep femoris) attaches to bottom of your pelvis as shown:
For most people, as you drop down in your squat position your hamstrings will pull on your pelvis and cause your lower back to round. That depth: where you are "bouncing" off your hamstrings", is the typically recommended bottom depth. Your spine is neutral, you've fully engaged your hamstring, and your knees are healthy.
Interestingly enough, the squat is actually a terrific hamstring and hip stretch for the above reasons. If you're squatting deep, you can probably put your fingers (and maybe your knuckles or palms) on the ground while keeping your legs straight.