I will try to be as brief as I can be. I'll address both legal and illegal immigration, as well as refugees in the following paragraphs. I'll start out by saying that no country can long survive being balkanized - and that is the inevitable result of mass-migration. This is not to say that immigrants are inherently immoral, or bad people. They're not. What I am saying is that without a common cultural starting point we will view the world in different ways, and thus we will interact with it in different ways - often having conflicting views on how to run the shared state apparatus. This does lead to conflict and animosity on all sides, particularly within large blocs/communities - as their numerical strength bolsters their will to shape society around their own ideals. This is just human nature and something that all people and groups of people do. The more influence you have the more likely you are to try to change things around you to suit yourself. Given this, I do not feel that we have any moral imperative to accept ever more immigration, especially from communities that are largely (Key word, largely) not integrating. Furthermore, more people means more city/traffic congestion, more pollution, a greater strain on water and food resources, and more tax beneficiaries who haven't paid into the system. I get that there are good reasons why many people leave their homelands, but that does not mean that it is our best interest - or in the best interest of the countries that they leave behind - to move here. Consider this: if we're taking "The best and brightest", does that not leave their homeland less prosperous and necessitate more foreign aid? If we're not taking the best and brightest, then how does it help us? Furthermore, what is wrong with people who lament/resent that their home towns and cities bear little resemblance to the ones of their childhood?
Those who oppose large scale immigration are often lampooned with the phrase "Dey took are jerbs" (Sic), attempting to mimic the accent of a low class southerner - in order to convey the idea that it's actually about racism, while is hiding behind legitimate social and economic concerns. That is, if they are not immediately characterized as some kind of supremacist or rabid hate-monger. However, in many states that stereotypical phrase about jobs actually bears fruit. In the 2007-2017 timeframe, 77% of growth of the labor pool of Tennessee was attributable to native citizens. Despite this, virtually all of employment gains went to immigrants . Furthermore the rates of welfare use is far higher in immigrant households (51%) as opposed to native households (30%) . So while an open borders advocate might scoff at the idea that they both take jobs, as well as use welfare at a disproportionate rate - it does actually bear out that way. Furthermore the presence of a larger pool of labor also provides disincentives for employers to raise wages/benefits, as there is less competition for workers. While not the only culprit behind stagnant wages, we'd be foolish to ignore the impact that it has had on American workers - particularly low-skilled workers, and those who rely on jobs doing manual labor .
However, that is to focus mainly on legal immigration. Illegal immigration is another problem in and of itself. Illegal immigrants are a net drain on the economy, providing a deficit of $14,000 per household. In the United States there are roughly 3.7 million households - which puts the total deficit at about $54.5 billion dollars. This includes benefits for children of illegals who gave birth within the borders of the USA, public education costs, as well as costs to public services (Police, fire, park services, and highways). These services are largely unpaid for. A common argument is that if we give amnesty to these illegal immigrants, then they will become a net gain for the economy, however, given that the majority of these people are below the poverty line it is estimated that they would go on to consume an extra $14,000 per household, putting the total deficit at around $28,000 per household. They'd also now be eligible for social security benefits and medicare. Amnesty would therefore have a short term mitigating effect on costs, but then once the interim period is through would lead to an explosion in costs. This would also reinforce the precedent set by Reagan's deal that one can break US immigration laws, and if you do it well enough you are entitled to services paid for by US citizens.  Furthermore, amongst illegal immigrant communities crime is astronomical - and likely much higher than what we know since many victims who are also illegal are hesitant to report crimes to the authorities  The worst part of this, is that if the law was upheld then these crimes would never have happened.
This is to say nothing of the changing collective soul of the United States, where US teens and young adults will label any common sense fixes to these above issues as "Fascism" and "Bigotry", leading to them marching in the streets and throwing rocks and bottles of urine at their fellow citizens in the name of protecting the "Rights" or even the feelings of people who do not belong here. None of this is to say that these people who are here illegally are deserving of scorn and hatred, as individuals. Nor is it to claim that they're inherently bad people. It is to say that their presence here is not good for US citizens, nor the countries from which they hail.
Edit: for anything with a citation number, just ask for that specific site and the name of the article. I did have it all nicely sourced, but I saved the original post in word.