If you set the MAYA_PLUG_IN_PATH in your Maya.env file as described in Setting up your build environment, plug-ins in the top-level of that directory will be loaded into Maya automatically. However, plug-ins that are not in that directory's top-level will need to be loaded manually.
To load a plug-in into Maya,
Open the Plug-in Manager. Click on Windows > Settings/Preferences > Plug-in Manager to open the Plug-in Manager.
Click on Browse in the Plug-in Manager.
Navigate to the location of the plug-in and double-click on it.
On Windows, samples are built to the build\Debug\ directory under the sample name. Windows plug-ins have the mll suffix. For example, to load the Windows circleNode plug-in, navigate to circleNode\build\Debug\ and select circleNode.mll.
On Linux, samples are built to the build directory under the sample name. Linux plug-ins have the so suffix. For example, to load the Linux circleNode plug-in, navigate circleNode/build/ and select circleNode.so.
On macOS, samples are built to build/Debug/ if they were built with XCode. Otherwise, the sample plug-in will be located in the top level of the build directory. macOS plug-ins have the bundle suffix. For example, to load the macOS circleNode plug-in built with Xcode, navigate to circleNode/build/Debug/ and select circleNode.bundle.
Plug-ins added manually will not be loaded into Maya automatically. They need to be reloaded into Maya every time Maya restarts.
If you want your plug-in to be loaded automatically, select its Auto load option.

This will bring up the plug-in's information, including whether it is a dependency graph node or a command, and for which version of the API it was built.
Important: Plug-ins need to be rebuilt for each new version of Maya and its API.