The Washington Post has a recording of acting White House chief of staff Mick Mulvaney addressing a private gathering in England this week on a variety of issues. The bit on immigration:
"We are desperate — desperate — for more people," Mulvaney said. "We are running out of people to fuel the economic growth that we've had in our nation over the last four years. We need more immigrants."
Many readers, and even the WaPo reporter I spoke with, were surprised by this, but they shouldn't have been; Trump himself has been calling for increased immigration for over a year now. He started his immigration-expansion tour at the 2019 State of the Union, saying "I want people to come into our country in the largest numbers ever, but they have to come in legally." (Emphasis added.) The following day he doubled down, saying "I need people coming in because we need people to run the factories and plants and companies that are moving back in. We need people." He's brought it up a number of times since.
Mulvaney was simply repeating what his boss had been saying for some time.