From the Trump rally in Erie, PA:

Under my leadership, we achieved the most secure border, Southern border in U.S. history. So we're building 10 miles—you know the wall is going to be completed very soon and we had radical opposition to the wall, radical opposition. We'll be at 500 miles very soon and we'll be completed with the wall.

And by the way, Mexico, you know they keep saying Mexico is paying for the wall, they understand that. They're paying for the wall. Mexico's paying for it and it's very high, just like I said it would be, and it actually goes down very deep. Do you know why? Two reasons, actually, number one, to hold it up.

Number two, to stop the tunnels. My opponent's immigration plan is the most extreme proposal ever put forward by a Presidential candidate. Biden would eliminate U.S. borders. How about that? Do you like that idea? [Audience Boos] OK, just think of it from a break—how the hell do you have a country if you don't have borders?

Note that fence construction is ongoing, despite GOP legislative failure to fund it, and various kinds of lawfare to stop it.

Also, Biden says if he gets in, no more wall building. He doesn't say he'll tear it down.

"There will not be another foot of wall constructed on my administration, No. 1," he told NPR's Lulu Garcia-Navarro during an interview with journalists from the National Association of Black Journalists and National Association of Hispanic Journalists.

[Biden Would End Border Wall Construction, But Wouldn't Tear Down Trump's Additions, August 5, 2020]

That doesn't mean President Harris won't tear it down, or allow Mexicans to tear it down without opposition, as if it were a statue of George Washington.