Details of previous iMacs I have owned?

Your iMac's serial number and usually EMC number are on the underside of the stand. If you have one of them you can look up the specs on everymac.com as @dialabrain already indicated.

You can also look up iMac specs by model, year, etc. here -> Identify your iMac model - Apple Support

if its still tied to your account then you can read it here

View and remove your devices that are associated with your Apple Account for purchases - Apple Support

otherwise you probably have to look for an old invoice from when you purchased it

If you still have it and it's old enough you can find some detail from https://everymac.com/ultimate-mac-lookup/