They are called "Drop Ship Companies."
Basically, "TEMU," and other online stores like them, take your order, then give that order to a wholesaler in China, and that wholesaler fills the order and sends the ordered item directly to the consumer. "TEMU" doesn't have warehouses nor inventory to buy or carry, so it is a very inexpensive business to run. The biggest problem to face is orders being filled wrong, returns, and refunds.
The profits are huge though, if you price your products right you can charge $19.99 for an item that costs only 1 or 2 dollars from the wholesaler.