yeah it's roughly the same with what @nostalgicroxas explained about open/closed species so if there's adopts open for that species it's sometimes limited to the world the artist made those species for
Basically just that each artist, and even within an artist's creations, might have different rules set up for different adoptables set.
A leafy adoptable might be Open, free to use however you want, credit where and when needed ect
Limited, free to use how you want but was designed for this world and shouldn't be used outside as the characteristics don't line up, usually permissions needed for ... wider usage needed
Closed, super limited, and likely even if you pay extra, you will not be able to use them as however you want, and severely limited in Rights/Copyright Transferance ect
An extra point, sometimes it's a species created by the artist and people respectfully pay them to get new characters. There's homebrew Aasimar variations, for example, or a cute critter that got really popular on social media. People won't use them outside of the main purpose, but you're paying for a commission that's instantly delivered, which is fun! (I just wish I was good at selling adopts :'D)
I don’t necessarily think they’re a scam. I’m just merely trying to understand them. To be honest, I’m looking for new avenues to sell art and stuff, and I keep seeing people selling adoptables. I looked up some stuff on them online and there were videos and other things about people saying adoptables were iffy or could be scams. So I wanted to see what other artists on here thought. Thanks for your insight. It’s very helpful. 
By “highly priced”, I’m talking about people unfairly pricing, like using bases for multiple designs, where the character isn’t well drawn, or you can tell it took half the effort good artwork should have taken. I do agree that artists should sell based on their time and effort, and how they value themselves.
Painting the entire community evil is not what I meant to do with this post. I apologize if it came off that way, but that was not the intention. I was just wondering about them because of what I’d seen online, and I trust this forum, being full of creators and artists, more than others just saying things on different websites. My asking about if it’s a scam wasn’t to directly say it’s a scam. I want to get into adoptables and am trying to figure out more about them. Maybe I should have phrased my topic post and question better.
phrasing definitely may have helped your case so i apologize for jumping to a conclusion. As for online videos some of those tend to intentionally go for discourse framing, cherry picking some of the worst cases or making sweeping assumptions or generalizations (one of the reasons I try not to turn to those too often).
But I will say some of the replies here kind of cover what your looking for with explaining what adopts are, that being character designs offered for sale or for bidding (the latter depending on demand and quality). I've seen them sold on sites like deviantart, twitter, maybe instagram, but most definitely FurAffinity. I imagine there might even be discord servers for adopts as well to find buying and selling circles (though i wouldn't know personally). Pricing can vary widely so it's good to research and see how other folks are pricing their work taking into account the style, level of detail, type of adopt and so on. But I hope you're able to figure things out (and apologies again) 
No worries! Yeah, I think I should have prefaced my topic with more detail as to what I was intending. Yeah, everyone’s replies have been super helpful, and I’m really grateful for your insight and everyone else’s. Thanks for taking the time to share what you know! I like asking questions here because there’s Such a good variety of creators here, who know about stuff that I don’t.
You also know exactly what it's going to look like, and generally if it's a character the artist was going to use, you also gain access to any other art of that character. So you immediately go out and start creating a little story for the character and buying commissions of it with an existing image. 
It's like getting clothes from a store--you see the clothes you want, you buy the clothes. The pricing of the clothes depends on how well they brand themselves and how good their work is. Not sure why you mentioned scam in the original post--other than maybe being scammed from a person saying they'll buy their work and then not?
Adoptables are kinda like clothes for RPer's, from my understanding. It does open yourself up to getting your work stolen, but at least one good person is out there buying these things because the adoptable community is alive and active. I personally don't do it, I prefer to sell my art other ways, but it works well for some.
Yeah, I mentioned scam because others had mentioned it over other websites, or seemed really put off towards adoptables. To be fully honest, I wanted to do some investigating because I want to start making adoptables. Mainly because I make so many designs and never actually use them. But I wasn’t sure if they were rip offs or not. (I couldn’t see why they would be scams or anything, so I took to these forums to see what everyone else had to say.)