Are Pre-installed OSX available on new SSD?

As @vladaad stated drives don’t come with the OS installed you need to install it your self.

There are a couple of ways to install a fresh OS with very little effort! If you have a good internet connection you can install the OS via Internet recovery just by connecting your system to either a WiFi or Ethernet connection.

If you have access to a second system you can setup your Mac Mini in Target Disk Mode and using a Thunderbolt cable between the two systems you can use the other Mac OS installer to install the OS onto your Mini.

Another way is to get a USB thumb drive and set it up on the other Mac or before you remove your current drive set it up as an OS installer.

So how do I do all of these different ways? Here you go!

Internet Recovery - Restart your system and press Command (⌘) and R keys

Target Disk Mode - Restart your Mac Mini system and press T key, then connect your other system to yours via Thunderbolt Cable.

Setup a bootable OS installer follow this guide How to create a bootable macOS Mojave installer drive

And here’s how to get the best macOS for your system How to upgrade to macOS Mojave (jump down to Step 4 for the direct link) Use this OS for either method.

References:

• How to reinstall macOS from macOS Recovery
• Mac startup key combinations
• Understanding the Applications for Target Disk Mode