Please note that linking blue lines with interior routes is not allowed in Google Maps. This feature was previously available, but is no longer used.
Only your blue line created on a street/road in GSV mode can be connected to other blue lines. GSV mode is a video mode, not picture. You record a video by driving/riding/walking in a special GSV mode and submit it to Google as video.
They strip frames from it at specific intervals and create a blue line from it. Google had this feature for inside tours that allowed to go from a blue line into somebody's place inside tour but it has been long gone. Occasionally an inside tour can be linked to a blue line but it happens only by accident.
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