understanding windows 10 updates: version, quality, os build info

How does one tell if a win10 is fully updated, or patched?

Hi, whilst the latest available build number for the general public and for different forks of the Insider program are maintained in the News section here, for example, that does not mean that each and every PC will be offered updates or upgrades to that latest build by Windows Update.

That occurs for at least these major reasons:
a. Compatibility: a given PC may not be deemed compatible with a given build and so not offered an upgrade to it.
b. Primarily considering upgrades (feature updates) - MS has a planned slow roll-out program, where devices considered the most compatible receive it first. Build roll-outs may be spread over 2-3 months, for example.
c. It is possible to defer feature upgrades for up to 365 days. That will delay upgrading, and Windows Update will not offer any available upgrade during that period.

In Settings, Windows Update, you can see when the last check was run, and run a check manually.

How does one find out what versions and quality updates are out there?

See the News section here

Does a later version such as 1909 with no quality updates beat out and any older version with N quality updates?

It depends. For example, I upgraded to 1909 from 1903, and rapidly found the widely reported problem with file explorer search. I reverted.

Quite often MS changes in feature updates- in this case, even the more trivial 1909 build- introduce new problems which may be fixed in the next major update- or the next feature update as seems will be the case here.

Every production version (1803, 1809, 1903, 1909, etc.) has a unique number to the left of the decimal point for that particular version. Notice the number in my system specs. The number to the right of the decimal point normally gets assigned when a Cumulative Update (typically the “B” or Patch Tuesday update) is applied to the system. So, a Feature Update will get your system a new number to the left on the left side and a Cumulative Update will change the right side. The current version numbering methodology follows a YYMM format as MS is trying to do feature updates twice a year (March & September). There are some anomalies to all of this for the folks in the preview groups, but if you are just a regular user, you needn’t be concerned about that part. If you are using the Windows Update function, the system will normally provide your particular PC with the updates it needs and can support.