It really varies by item, OP. I buy from Zara a lot and it just depends. A coat we bought lasted 2 years beyond when DD was wearing that size, but we buy their sweatshirts a lot and always have to size way up to get the intended oversized fit. But I’ve found pants/leggings/ dresses to run pretty true to size.
When I order online, I size up one or two sizes and hope for the best. When I can, we go in store so we can try on.