Files missing in iCloud - folders there, but empty

I was browsing through my old documents and I am discovering many files in my iCloud are gone. I have the structured folders, but there is nothing inside them. I have no idea what has gone wrong but I am incredibly disappointed. I may be able to recover some from old USBs with the information. I need to understand what has gone wrong so that it doesn’t happen again. It seems that files may not have synced or something? I am at a loss but so upset at what a terrible product iCloud is. Did this happen to anyone else?