Hello,
First off, let's tackle the problem of the "California Complete Photoreal" not showing up in your scenery_packs.ini file. This issue usually arises due to the simulator not recognizing the new scenery content.
The simplest workaround is to manually add the "California Complete Photoreal" to your scenery_packs.ini file. You can find this file in your X-Plane 12 directory, inside the 'Custom Scenery' folder.
Here's the syntax for the manual entry:
[indent]SCENERY_PACK Custom Scenery/California Complete Photoreal/[/indent]
The simulator parses the .ini file in descending order. Thus, the priority is based on the order of the entries in the .ini file - lower entries have a higher priority. So, you should put the "California Complete Photoreal" entry above other entries to give it a higher priority.
Next, let's discuss the XP11_UHD_Mesh_V4_USA_West_3 scenery. This is a detailed topographic mesh for the western part of the USA, improving the realism of the terrain. It will make the underlying terrain for your "California Complete Photoreal" more accurate.
Yes, it is beneficial to have this. In your scenery_packs.ini, you should place the UHD Mesh entry below the Photoreal scenery. As a result, X-Plane will first load the photorealistic texture from the "California Complete Photoreal" and then overlay it onto the accurate topography from the UHD Mesh.
Here's how your scenery_packs.ini file should look:
SCENERY_PACK Custom Scenery/XP11_UHD_Mesh_V4_USA_West_3/
SCENERY_PACK Custom Scenery/California Complete Photoreal/
Once you've made these changes, save your scenery_packs.ini file, and then try restarting X-Plane 12. The simulator should now recognize your new scenery files.
As a side note, X-Plane 12 and Microsoft Flight Simulator 2020 have different ways of handling scenery files. The method I've described applies specifically to X-Plane 12 and may not be suitable for other flight simulators.
Hopefully, this helps you get up and flying over a more realistic California. Should you need further assistance, don't hesitate to reach out again!