MacOs Install issue

Something is not right in your description. Here is a brief description of how to do what you want.

• Download the OS installer file. It should not be done via W-Fi as it will often fail and the file can be corrupted. if possible, use a wired Ethernet connection. If not possible, multiple downloads may be needed to get a good one that will work.
• The download to your computer's HD can take up to 45 minutes. Then the installation on the SSD can take another 45 minutes with multiple automatic restarts.
• Once the OS installation is complete, you will then need to complete the setup process for the new operating system.
• Finally, you can open System Preferences and select the SSD as the startup drive and restart the computer.
• You cay "Once it restarts the ssd shows that a macos installer it there but i can’t boot into it". Something wrong here. The OS installer file would remain on your Mac's HD in the Applications folder. (It can now be deleted.) The SSD would now have an operating system on it, ready to be set up. As noted, you should be able to open System Preferences and select your desired startup drive at this point. Then do a restart.

Is this the process you followed or was it something different?

Which exact iMac do you have?

Are you aware thatApple does it's best toprevent you or I from installing an OS older than the one the Mac came with originally?