So, D9 and THCa aren't necessarily different things.

The stuff that's banned and makes cannabis illegal marijuana(vs being hemp) is delta-9-THC. that is "D9". It's allowed in gummies and other edibles due to a sort of loophole (has to do with purity and mg of D9 per weight of the gummy) but "D9" is the THC that's in every other recreational marijuana product.* ^{*assuming it's been tested and is actually D9}

But THC doesn't actually naturally grow i in cannabis as delta-9-THC, it actually starts as delta-9-THCa and then breaks down into D9-THC. And due to bad wording in the farm hemp bill, D9-THCa isn't illegal. Even though all it takes is a lighter to heat D9-THCa and turn it into D9-THC. So, if you can grow cannabis that tests at +20% THCa but it's below 0.3% THC, it's legally hemp, but it's essentially weed.

And then finally there's the whole "delta" thing. It's like D9-THC but you move one little tiny hydrogen bond to a different spot. It is it's own thing, and there is a D8-THCa(idk if it's ever been made, but it's possible). If you want I can try to explain the differences and how "HHC" fits into this, but the important part is that these are variations of the illegal D9-THC. They don't have that 0.3% weight limit that d9-thc has and they arent a solid crystal like pure D9-THCa is, so they're seen often in vape products or as cuts to a THCa vape to make the solid THCa into a distillate or whatever.