Can you elaborate on what you’ve noticed with respect to the “differences in quality”?
That's a fair question. Here's a bit of my experience with various brands. Of course, all this involves a sample size of just one or two rifles, so your mileage may vary.
Aero Precision: I had high hopes for AP. I loved the finish on the ones I looked at locally, and I got a deal on a midlength carbine from Brownells in mid-2016. True to expectations, mine had an excellent finish, in fact one of the nicest finishes I've ever seen on an AR. Unfortunately, the the plastic handguards were molded poorly, producing sharp edges that needed to be lightly sanded. That's no big deal, especially since many will only replace the basic handguard anyway. Unfortunately, the upper had a canted front sight, which I'd only seen before on AKs. Even with optics, the barrel was poor. Groups from a rest with the AP were larger than standing offhand groups from my other rifles.
Model 1/Olympic: Many years ago, when I was just getting into ARs and there weren't nearly as many manufacturers on the scene, I built two rifles using Model 1 kits on Olympic lowers. The pin holes on both lowers were apparently out of spec, because I consistently had issues with walking pins. One rifle also had a defective trigger return spring. The other had a chipped firing pin. Neither would run steel-cased ammo until I replaced the BCGs with new ones from Daniel Defense. One positive was that the E.R. Shaw barrels used by Model 1 were pretty accurate.
Rock River: This was also a while back. I recall that it was generally OK, but I also periodically had stoppages. I remember the aggravation, but I seriously cannot recall the specific issue.
Spike's: I had two of these. Both were superb, and I should have kept them, but I downsized a number of years ago and both Spike's rifles went out my door.
CMMG (not mentioned previously): This one was a dedicated 9mm carbine. It was phenomenally accurate. However, I had quality issues with both the trigger return and bolt hold open springs. The receiver extension (buffer tube) was also out of spec and did not properly capture the buffer retainer pin even when fully screwed into the lower receiver. Unfortunately, I learned this when the retainer pin came loose during a range session and jammed my BCG half in and half out of the receiver extension. That was a fun fix!
Colt/LMT/BCM: Not a single problem with any of these brands, EVER.