Your squats don't look bad, but your narrow stance and excess depth are reducing the musculature involved in the movement, and this may be why you're not getting much of a bounce at the bottom. I've had to put people in narrower than usual stances sometimes, to solve other problems or because of anatomical issues, but it's not optimal if it can be avoided. Try a slightly wider stance and go to just below parallel, then drive hips up from there.
WRT your question: yes, you brace your abs (NOT flex) after you take your big breath in, but this isn't an either/or with regards to arching. When you're standing at the top of your squat, you should already be in an 'arched' / extended position in the low back. We do NOT want you to exaggerate this position into over-extension. We want you to MAINTAIN this position as you go down and come back up. The erectors must work hard to do so during the movement, because the weight held on your posterior deltoids is trying to make your back go into flexion, so your erectors must combat this. So during the movement, you'll be actively working to maintain your normal arch, but do not go into purposeful over-extension at the top before descending (nor at any other time during the squat).