"These are not just trainers."
Translation:Це не просто кросівки.
"Це" is not only used as an definite demonstrative pronoun (where it indeed has to match the word it is referring to in gender, number and case), but also as a "can't-stand-alone word" with the meaning "that/this/those/these (is/are)".
As you can see the number of the word it is referring to doesn't even matter now. This can get confusing because in English the words "this/that/those/these" always have to match the word they're referring to.
Think of it that way: "Це не просто кросівки" --> "These aren't just sneakers". Because "це" doesn't act as a definite demonstrative pronoun here, "просто" only referrs to "кросівки".
"Ці не просто кросівки" -->" These sneakers aren't just...". Here "ці" does act as a pronoun and therefore "просто" referrs to both ("ці кросівки").
As you can probably tell, the sentence is incomplete now and doesn't make much sense. I know it seems hard to understand, but "це" (and only "це", not other forms) is used so frequently, it will come to you eventually.