Older mac cannot be updated beyond High Sierra 10.13.6. The IPhone IS detected for photos, but not in Finder.

Finder never detected an iPhone before macOS 10.15 when iTunes app functionality was replaced by Finder.

With 10.13.6, except for importing photos, iTunes was used with an iPhone instead of Finder. See: Use iTunes to sync your iPhone, iPad, or iPod with your computer - Apple Support

ymm424 wrote:

So, in your opinion...must I worry about buying a new desktop now?

Budget for a replacement happening, certainly.

You’re on decade-old hardware, and four years and an architecture back, and prolly still on a decade-old HDD (that’s old), and are no longer getting security patches and updates.

Various app vendors have a most-recent-three-versions policy, and Apple can require the most current version.

The iMac 21.5” was replaced a while back with an Apple silicon M1 model iMac 24” model, and we’re waiting on whatever might replace the iMac 27” model.

With this decade-old hardware, keep current backups with Time Machine or otherwise running, too. Old HDDs can tend to fail by getting slow and showing beachballs, then wedges and hangs and app crashes and data corruptions, and then complete failure. Your data is difficult or impossible to replace without backups.

ymm424 wrote:

Thank you for the response. So I don't have to dump my oldie but goodie desktop? lol

I was trying to put ibooks (now called Books) to my iphone and this is where the problem began. I figured out how to do it with icloud however.

So, in your opinion...must I worry about buying a new desktop now?

If it still works, it still works. When my version of MacOS changed, all the iOS iTunes backups were copied over to Finder. They did a lot of stuff too including eliminating any of the iTunes directories, but they end up somewhere.

As FoxFifth said, just use iTunes. macOS Mojave and earlier use it for syncing.