To distinguish them from monthly updates (that fix security issues and other bugs), Microsoft calls each 6-monthly download a “Feature update”. I think other people sometimes call it an “upgrade” as another way to make it sound more substantial.
It took me a while to realise that the numbers refer to year and month, e.g. “1803” means March 2018. They seem to have decided that this number will only ever refer to March or September, even though they are released (for English) about a month later.
Actually, I wasn’t aware that they were released even later for other languages. I’ve used a program called WSUS Offline Update to download Windows updates, and, while for some versions of Windows and Office you have to download the updates for each language separately, the downloads for Windows 10 are “Global”. That said, WSUS Offline Update only downloads the monthly updates, not the feature updates, presumably because Microsoft provides their own tool for installing the latter offline. So maybe the feature updates are language-specific.
All the tech sites that I’ve looked at on this issue recommend delaying these feature updates because it takes Microsoft time to iron out the bugs in them. If you have Windows 10 Pro, then you can do this in the Settings app (that’s what the new-look Control Panel is officially called). It seems that Microsoft’s policy is that if you haven’t paid the extra for Pro, then you pay instead by being the guinea pigs – being the first people to trial each feature update.
Not sure why you didn’t get 1809. If you want a Feature update before Microsoft rolls it out to you (as I said, not recommended by most tech websites), then checking for Windows updates manually is the way to get it. I’m not sure if there’s a way to get 1809 from Microsoft now that 1903 is out, but you might be able to download the ISO from a third-party site.