Mainly walls are needed in high population areas...

They have them there already. San Diego. Nogales. El Paso. The problem really is the rural areas, which are some of the most remote in our nation, and are difficult to patrol or surveil (especially without funding). Someone intent on getting in will just go outside the urban areas, and even if patrolled by drone, once they cross you have to give them due process. We need more resources to do that efficiently, and remove those who need removing.

By the way, I've driven along part of the border. Very remote outside of Yuma. There was wall/fencing clearly visible from the highway, and we weren't anywhere near a city (many miles). People will find a way (over, under, through, around). So, while physical barriers strategically placed may have some benefit, streamlining processing of migrants (and especially asylum-seekers) will do a lot more. I think most of the really useful physical barriers existed even before Trump.

We wouldn't have to release them into the country pending hearings if we had more resources. If the POEs were better-staffed, we'd process people faster so they wouldn't feel the need to go over/under/through/around, which would reduce the traffic of those who do, making them easier to detect and detain (i.e. if everyone is going to a POE, someone that isn't stands out like a sore thumb).