can I use old iMac as second display?

Use your iMac as a display with target display mode - Apple Support

"You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display."

iMacs are computers and are not external displays for other devices, including Macs. Please buy and external display for your new iMac. They are inexpensive, simple to setup and use and are reliable.

DuetDisplay and LunaDisplay are two 3rd party apps that will allow you to use an iMac as a second display.

I hope this helps.

Paul