iMac not displaying all devices

In Finder, check that you have enable the display of these devices.

See the devices connected to your Mac - Apple Support

Check Finder > Settings... General tab and Finder > Settings... > Sidebar.

Follow up –

Make sure you are using known good connecting cables.

And if the devices that aren't showing up are connected to the Mac by means of a hub or dock, try connecting them directly to the Mac. You may need to use a USB-A to USB-C adapter or cable for this.

if the mac is connected to the same router as the ios devices then it's a question of you bridging the wifi networks and the wired Ethernet in order for them to be located on the same network, and of cause if the ios devices is not connected on the wifi at all then these things do not work across the internet

how one goes about bridging networks in ones router differ from router to router a google for

<Insert wifi router name model> bridge networks

should provide some guides