iMac disk erase and OS

How to reinstall macOS - Apple Support

When you install macOS from Recovery, you get the current version of the most recently installed macOS, with some exceptions:

• On an Intel-based Mac, if you use Option-Command-R during startup, you might be offered the latest macOS that is compatible with your Mac.
• On an Intel-based Mac, if you use Shift-Option-Command-R during startup, you might be offered the macOS that came with your Mac, or the closest version still available. 

There are several factors that determine which macOS version Recovery Mode will install.

carefully study > Use macOS Recovery on an Intel-based Mac - Apple Support

The late 2015 iMac can run up to Monterey.

If Recovery Mode installs the original or an earlier macOS version,

then you can get Monterey by upgrading or from > How to download macOS - Apple Support

Side note:

The above only works if the previous owner properly prepared the iMac before selling it to you.

If it was not properly prepared, then you need to contact the seller and return it for a refund.

see > What to do before you sell, give away, or trade in your Mac - Apple Support

Standard recovery mode should offer you the newest OS your Mac can support or the OS your Mac originally came with. I don’t think it will give you a choice, it will be one or the other.

You're welcome.