Your first call was right, only you need the snippet as well to get the video IDs.
https://www.googleapis.com/youtube/v3/playlistItems?part=id,snippet&playlistId=PLrEnWoR732-BHrPp_Pm8_VleD68f9s14-&key=KEY
Put the above in your browser to see the JSON result. You'll want to grab the
snippet->resourceId->videoID
for each item. Once you have the videoID you can make the url by just adding it to the end of the string "https://www.youtube.com/watch?v="
I think that's what you were asking for.