To check which macOS version is installed: 

From the Apple menu  in the upper left corner of your screen, choose “About This Mac.” You should see the macOS name, such as macOS Sequoia, followed by its version number. 

Which macOS version is the latest?  How do I install the latest version?

When a major new macOS is released, it gets a new name, such as macOS Sequoia. Software updates to a macOS are designated with a latest version number.

As updates become available, the table found on this Apple support page is updated to show the latest version of that macOS (you can also scroll to the bottom of this page to find out whether your Mac is compatible with the latest macOS versions): Apple macOS/Latest Version Table and macOS Compatibility

If your Mac is using an earlier version of any Mac operating system, you should install the latest Apple software updates, which can include important security updates and updates for the apps that are installed by macOS (i.e. Safari, Books, Messages, Mail, Music, Calendar, and Photos). Instructions for installing the latest software update can be found in this Knowledge Base article: How to Check for Software Updates on your Mac Operating System