In a macro sense, immigration (of any sort) is just lost in the general annual population growth. In the micro sense, concentrated immigration has acute effects since the local population change is much higher than you'd get from birthrate/deathrate changes.
It's actually a bigger chunk that you'd think. US population growth is about 0.7% a year, or just over 2 million. Immigration is about a million a year. Population growth without immigration would be a mere 0.35%. However, average fertility is only 1.77 babies per woman. This is not actually that much more than Japan (some states such as Massachusetts are around 1.5, which is a Japan-tier lack of babies).
American natural-born generations are already shrinking, but the decline is hidden because there was a big increase in life expectancy after WWII. Wait for the baby boomers to die off and you'll start to really notice the decline. Immigration is needed right now not to "grow" the population but just to stabilize the size of future cohorts.
BTW You know those "why do Japanese young people not have sex" articles? Those are a red herring from the Japanese press because mainstream Japan doesn't want to discuss the real issue - lack of immigration, so they blame "young people with their cartoons and video games" for the lack of babies. It should be easy to see how dumb this argument is. Massachusetts is the same as Japan, but would you accept an article blaming Fortnite and Spongebob for the lack of baby-making in Massachusetts?