So how do frozen embryos work?

Sorry if this sounds stupid but it's something I've never understood.
Do they continue to multiply cells as they're thawing? If so, if they were 8 celled and grade whatever when I last heard about them.... what will they be when they put them back? Are they still 8 celled or is it more like the equivalent of doing a 5 day transfer, vs. a 3 day one? Does this even make sense? Sorry--- my mind is overthinking today. TIA!