Download Encore.
Two apps, free during early access: one for your Mac, one for the old iMac.
This Mac runs macOS 10.13, so it's the iMac: download Encore Receiver here.
This Mac runs OS X 10.10, so it's an older iMac: download Encore Receiver Legacy here.
For the old iMac
macOS 10.13 High Sierra or later. Intel iMac, Late 2009 to 2020.
For an iMac from 2007 to Early 2009
OS X 10.10 Yosemite, 10.11 El Capitan or 10.12 Sierra. Like the 20- and 24-inch aluminium iMacs.
Version 0.9.0
Both apps update themselves from 0.5.0. What changed in each version.
Next
macOS warns once the first time you open the apps. The install guide shows the clicks, or use the Terminal line there.
Old browser on the iMac?
Open encore-ivory-omega.vercel.app/imac on the iMac. It's a light page with one button.