Without knowing exactly what type of AR marker you are interested in, I'll talk about two types that I am familiar with: ArUco and April Tags. Both are AR markers that have open source libraries with stripped down (and possibly outdated) versions implemented in OpenCV.
These libraries will give you the full pose of the camera based on the marker in the field-of-view. The library uses the physical marker size and camera calibration parameters to estimate the pose of the camera w.r.t an individual marker or set of markers. You may use multiple markers in the form of a marker map to enhance the estimate of the camera pose. I imagine when people use multiple markers as in the Boston Dynamics example it is to ensure track continuity as markers enter/leave the field-of-view or to mark specific aspects of a scene.
If you are interested in using ROS, perhaps this aruco_localization repo will be of use. It was set up to localize a mini-quadrotor with an upwards facing camera, as seen here.