It can get a bit confusing. The below website summarises the server released.https://docs.microsoft.com/en-us/windows-server/get-started/windows-server-release-info
There are two servicing models for Windows server - a long term servicing channel and a semi annual channel.
The Long Term Servicing Channel (LTSC) is like the old server releases of 2008, 2012 etc. They are major releases that are supported for a long time. While they do get security updates, they don't get many (if any) feature updates. The idea behind these version of Windows Server is that it is stable, so it's a good choice for your core infrastructure.
The other servicing option is is the Semi-Annual Channel (SAC). This version of Windows Server has new features, but a much shorter support period. The new features tend to be geared more towards things like Containers and Microservices - devops stuff. This version of windows is unavailable with a GUI - it is in core mode only.
Unless you need any of the features in the SAC branch (1909, 1903) then there isn't really a need to upgrade. Nearly all of our clients are on 1809 because it is stable - only a couple have deployed 1903 or 1909 and these are for very specialised reasons.
As 1809 and 1909 are two different servicing channels, they cannot be upgraded in place.https://docs.microsoft.com/en-us/windows-server/get-started-19/servicing-channels-19
Hope this information is helpful.