Here is one more piece of information, after I searched Apple support for idelassetsd, I got this
Use Apple products on enterprise networks - Apple Support
it says:
Apps and additional content
Apple devices need access to the following hosts and domains for installing and updating apps, using certain app features, and downloading additional content. Some additional content might also be hosted on third-party content distribution networks.
Hosts: sylvan.apple.com
Ports: 80, 433
Protocol: TCP
OS: tvOS and macOS
Description: Aerial screen savers and wallpaper
Supports proxies: —
I also played with LittleSnitch firewall to see the effect on Activity Monitor CPU %, and real-time network traffic (reported for process by LittleSnitch) when I block and unblock network access for idleassetsd.
Blocked: about 1,000 "network access requests denied" per second, CPU % usage goes to about 150% and system heats up, and network download is reduced by about 500KBytes/sec.
Allowed: one "network access requests allowed", CPU % usage goes to about 2% and system cools off, and network download is runs continuously at a rate of additional 500KBytes/sec over whatever else is going on.
Network upload traffic is negligible compared to download traffic.
Again, this is on Sonoma 14.4.1 on an old Mac mini (2018) on a rural DSL line that only achieves (at best) 4MBits/sec download and 512Kbits upload speed (often worse - Thanks to Brightspeed/Centurylink for never replacing the buried copper-pair POTS lines since they were put in).
At some point, I might get to transport the MacMini to a place with Google Fiber and let it download to its heart's content. Maybe I only have another 50GB of animated screen savers and backgrounds remaining to be downloaded, and it will be finished with me and this episode will end. Apple, you have a demand-hog failure with idelassetsd (150% usage thread and 1,000 network requests per second makes my system glitchy - how about backing that off to 1 network check per five seconds for this NON-ESSENTIAL daemon task?).