Basically the wall is being built where it can be built. As stated the real wall is going up as “upgrading/maintenance” of the existing wall. I will tell you that a lot of the existing wall needs up grading. So IMHO the wall is starting to be built.
At this link there are pictures of the existing “wall”. You can see where parts of the existing wall needs “upgrading”. Just upgrading the existing “wall” will cover a lot of miles.
http://www.foxnews.com/us/slideshow/2017/02/02/in-pictures-u-s-mexico-border-as-it-stands-now.html#