But how to understand which item is copyrighted and which is not?
It's pretty easy. On AliExpress, VERY VERY few of the companies are
licensed to sell/make any of the big names we all recognize.
Is it a famous clothing/shoe brand? It'll be copyrighted. (Nike, Adidas, Lululemon)
Is it a famous movie/tv show/video game? It'll be copyrighted. (Game of Thrones, Walking Dead, Pokemon, Harry Potter)
Is it a popular sports team or person? Copyright.
See a pattern...?
Any big (brand) name that a lot of people recognize around the world you should expect will be copyrighted. Does that mean you CAN'T sell it? No..., but you should be aware that with copyright comes lawyers and teams of people protecting the brand. There's always a chance you'll get told to stop. There's also chance you won't.