I've bought a lot of stuff from Temu (even a $150 laptop that I am actually quite impressed with. It's low-end for sure, but for what I needed it for it's perfect and new (so it runs Win 11). My old laptop was from the XP days, so needed a bit of an upgrade..)
I've bought lots of shirts (included a NMBC shirt I wore to DL...) I've found that I want to get a size larger than I normally wear, as their shirts seem to run a bit small...
As for authenticity, I've encountered two situations
1- seems to be the factory (or someone with access to it) selling the same official products cheaply. I got a 'Lucas the Spider' plush from them for dirt cheap, and it was the official licensed product that sells online for much more. (I assume I'm basically depriving the copyright holder of their profits though, so there could be moral issues there...)
2- Or, it's a factory selling a cheap knock off, and using official pictures to advertise their product. (There was a Sisu dragon from Raya on there, and they were showing pictures with obviously an official Sisu plush, but if you looked at the comments and pictures from customers, the item they were selling looked horrible, clearly just a 'we made one that looks like this picture', but did it blindfolded...)
So, before you buy anything, look at the reviews, especially ones with pictures of what they actually received. If there aren't reviews, either look for other listings of that same product, or just pass on it (or take the risk if it's really cheap and you want to see what they send)...
It is similar to Amazon in the sense that I don't think 'Temu' themselves sell anything. It's just a store front, and anyone can create an account and start selling things, so the quality can be all over the place... And, for many things, you will find them being sold by more than one person, and possibly with some price variance.
But with careful purchasing, I've only ever received one thing that didn't actually work, and I was able to return it (and they didn't want it back, just refunded my money and I threw it out).
A friend of mine uses it a lot too for trinket like things, and she's 'returned' a lot of stuff (she says you are allowed one return per order), and only once did they want the item physically returned... I would imaging cheap things like pins they would just tell you to keep and refund your money.
And also be aware that shipping can be slow, especially if you are used to Amazon. Sometimes it will ship from a domestic hub, other times it literally is coming straight from China, so it can take 2-3 weeks sometimes...
But again- the most important thing- look for customer reviews with photos! That's where you will see the truth...