How do I install macOS version 10.15.0?

Link-> Download Catalina 10.15

1. Click the link to open the App Store page for that macOS. 
2. In the App Store, click the Get button to begin downloading the macOS installer.
3. After downloading to your Applications folder, if done on a supported computer running an older version of macOS, the installer opens automatically. Follow the onscreen instructions to install macOS.
4. More info here: link-> How to download macOS - Apple Support

Phil0124 has you covered, just adding that 10.15.0 is not available, Apple only provides the full 10.15.7, or Updates if you have 10.12.0 to 10.15.6 already installed...

How to download macOS - Apple Support

https://apps.apple.com/gb/app/macos-catalina/id1466841314?ls=1&mt=12

How to upgrade to macOS Catalina – Apple Support

And caution on 10.15 or higher, to see which Apps will no longer work in 10.15 get Go64...

https://www.stclairsoft.com/Go64/