It looks like a fairly low skill exercise, but it isn’t and for some people getting the ‘feel’ of a decent deadlift is really hard. My suggestion would be to watch some coaching videos by respected people like Dave Tate etc and try to mimic that.
Yes its an exercise for back and legs, but your lower back should be essentially immobile - it gets worked by fixing you in position. A bent lower back is more likely to get injured.
Your upper back essentially does the same, although movement there is greater and more acceptable.
Knees and hips will bend, but it’s really a ‘hip dominant’ exercise, meaning the most movement will be at your hip joint, not your knee joint.
Try with feet slightly narrower than shoulder width, so that when your knees flare at the bottom, it all still fits between your arms. Grip the bar at about shoulder width.
You’re also looking very awkward trying not to touch the weight to the ground. It should touch every time. Your choice whether you let it bounce a little or reset completely between every rep.
Look at video of Jim Wendler doing deadlift reps, and then compare them to yours. Assess the movement critically and then try to mimic his as much as possible. He’s a good example of a strong raw deadlifter with good technique.
Whether you use straps or not doesn’t matter if you aren’t competing, but at the moment if you want to improve your technique I’d suggest backing off the weight anyway, so you may as well ditch the straps and improve your grip strength while you work on technique.