Pay and conditions were not destroyed because of immigrants, but are a side effect of globalization. Production facilities were offshored, leading to a collapse of local manufacturing and low skill jobs (where no prior education is required, low skill jobs can and sometimes do require a lot of skill). Reshoring those jobs is incredibly difficult because it also means reconfiguring the supply lines. For urban areas this is not a huge issue, as those can switch to a service based economy, but for small towns this often meant that the main economic driver of the town left and people lost jobs.

Anyway, not caused by immigration. As a matter of fact, immigrants are a significant driver of economic activity and partly offset that.