I've looked at quite a few crystals over time, and what I can tell you is that on those sites you get a real mix. Some stuff is actually natural crystals, just sold dirt cheap because it's mass sales with no middleman. But a good chunk of it is synthetic or artificially dyed, and sometimes you even get glass or resin knockoffs. The problem is you have zero certainty when you order.
As for the energy side of things, the person above was right that it's in the spiritual realm, so I'm not really going to weigh in on that. But on the material side, a real amethyst is a real amethyst no matter where it comes from, as long as it's genuine mineral. But a fake is a fake. Specialty shops at least generally control what they sell and can guarantee it to you. On Temu/AliExpress you're buying blind, you could end up with something totally different from what you ordered.
If you really want to know what you got, you can check some basic characteristics yourself (density, hardness, transparency) but otherwise you can also get someone who knows their stuff to test it. It's pretty much a gamble with these sites. It might be worth it if you're willing to accept that you could get anything in your order.