Many, if not most, of his loud boasts of controlling immigration, building walls etc never actually came to fruition.
I can only assume you were not paying attention when he had an agreement with Mexico to stop them. That was cancelled by Biden.
https://en.wikipedia.org/wiki/Remain_in_Mexico
Remain in Mexico (officially Migrant Protection Protocols) is a United States immigration policy originally implemented in January 2019 under the administration of President Donald Trump, affecting immigration across the border with Mexico. Administered by the Department of Homeland Security, it requires migrants seeking asylum to remain in Mexico until their US immigration court date.
The policy was initially ended by the Biden administration,