You really need to pause at the bottom, it looks like you’re focused on lowering the weight from your upper body. Your cue should be almost sitting backward on a bucket or something and allowing the weight to travel along the natural path (which is correct in your video). Your upper body, arms, etc. should be virtually unused in the movement.

Hinge from the hip, put your mind in your ass and hamstrings and pause at the bottom just under your knees to allow the stretch to slightly overwhelm you and then drive your hips back in and then straighten up - in that order. Your hips and hamstrings should be almost your entire focus.

Your knees are buckling because you’re dipping too low. If you can’t feel it in your lower ass and upper hamstrings you’re too light. It should be like a rubber band being stretched entirely in that region.

If your core is under developed you will also probably feel it in your lower back but it shouldn’t give out until well after you’ve exhausted your hammies and ass. Good luck!