boot from one os computer to another

Sounds as if you need to startup the 2017 iMac in macOS Recovery Mode, format the new SSD and Install macOS.

first see > https://support.apple.com/en-us/HT208496 to format the SSD,

then see > https://support.apple.com/en-us/HT201314 to Install macOS.

What is your question and please explain why you are asking it?

Also, please read Writing an effective Apple Support Communities Question so we do not have to play 20 questions with you.