> step one in population control is shutting down immigration

Yes, overconsume so much, that it heats the entire planet. Live in an area where is isn't as bad as elsewhere.

Build walls around borders to keep those suckers threatened with loss of livelihood to keep them our of our utopia.

Bad news, buddy.

Once those hundreds of millions/billions start moving, no walls are gonna stop them.

How does preventing immigration effect the global population?

Countries with high birth rates will hit their local maximum carrying capacity. Immigration allows it to "blow off". As a past example, there are about 100 million people of Irish descent in the world. This is many more than there would be had they been limited to Ireland itself, when Ireland's birth rate was very high.

I assume you are white and live in America. Seems a smidge hypocritical.

Also this doesn't make much sense. We live in a globalized world. Singapore, for example, gets 80% of its food from abroad. Their "carrying capacity" is not particularly limited, except for literal space. For a country-sized country, space is functionally unlimited.