Locating monument with OPUS vs locating CORS

The short answer is that CORS are located with respect to "backbone" global stations positioned in the ITRF.

Here's the NGS super-helpful explanation of the latest CORS solution/positions (with lots of info on how they perform updates and adjustments):

https://geodesy.noaa.gov/CORS/news/mycs2/mycs2.shtml

Here's a (slightly dated but really good) paper on investigating and comparing updated CORS positions with previous ones, versus OPUS solutions:

https://digital.osl.state.or.us/islandora/object/osl:10403

While each CORS will vary a bit in general NGS uses two weeks of data to ensure that any new station is stable in all three positional components (latitude, longitude, ellipsoid height) before they will allow the station into the network.  Once accepted each station in the NGS CORS network (2000+) is solved with a 24 hr data set each morning in a global network adjustment and that data is compared with the adopted position to watch for any motions and to ensure the integrity of the position.  NGS provides both short term (90 days) and long term (since inclusion into the network) graphs of how each station is performing.  Regrettably most users don't ever bother to look at these.  So easy to find -- go to NGS CORS web page, select and station and select "Get Site Info."  On the left side of the page the links for graphs are provided.

I miss the work I used to do with all these moving parts and adjusting and learning how to use the tools.

I'm ok where I'm at currently, just not getting the daily education and challenges in the geodesy, but other areas.

Go geodesy!