DevOps or Development Operations is the set of tools, processes and practices used to enable the ... well operations of your development effort.
At a minimal this will include some means of source control, change management, build and testing. Their are a large number of tools available to help you and your team ranging from free source control such as GitHub, to 3rd party tools like Perforce or Plastic SCM or more industry standard tools like Azure DevOps.
DevOps is a huge topic that can be a whole career for some people. Its also just a tiny part of a wider concept you should understand as a company or software engineer called Application Lifecycle Management or ALM. For hobbyist and enthusiasts ALM and the deeper aspects of DevOps is likely overkill, but everyone should have the basics of source control, change management, build and testing and you can get that for free with GitHub and Unity.
If you do want to learn more about DevOps and Application Lifecycle Management I would recommend starting with Microsoft's documentation, courses and tools on the subject. Love them or hate them Microsoft is the single largest actor in the software engineering industry as such they have a lot of good and free resources to get you started.