I have an iMac 2017 version with HDD and no fusion drive. HDD has slowed down so I am planning to use an external SSD. In my earlier question a generous user told me that in my model of iMac High Sierra will work better as APFS don’t work well. I just want to know that if I use an external SSD to install MacOS then can I install latest version or use only High Sierra.
You can install Ventura (macOS 13), which is compatible with your Mac. The latest is Sonoma (macOS 14), which Apple has stated is not compatible with your Mac. If your internal drive is running High Sierra or newer, then you should be able to download the Ventura installer application from the Apple App Store. A link to Ventura in the Apple App Store can be found here. You can use this application to install Ventura on an external APFS formatted SSD.
You can install Ventura to an external SSD without also having High Sierra installed on this SSD.