I find this a useful way to organise my thoughts about the costs and benefits of immigration. It may work for you too. I start out with a neutral benchmark, where immigration has neither costs nor benefits for the original population. Then I think of different ways in which that neutral benchmark could be wrong. This post is just a list (no doubt incomplete) of things that might create costs or benefits from immigration. I make no attempt to say which is bigger. It depends.
I am writing this mostly for non-economists. I should warn you that the economics of migration is not my area. I'm a macroeconomist, and most economists who specialise in immigration are microeconomists. This may give me a different perspective.
In case you think it matters: I migrated to Canada from the UK 40 years ago (and to Quebec from Ontario 30 years ago). This may influence my perspective.
And for what it's worth: I think that Canadian immigration policy is probably in the same ballpark as the right immigration policy for Canada. Though it is probably different for different countries.
The Neutral Benchmark. The country clones itself and becomes twice as big. Everything scales up in proportion. The supply of everything doubles, and the demand for everything doubles, so all prices (and wages) stay the same. Total output and income double, but output and income per capita stay the same. There are neither costs nor benefits for the original population.
I think that's a good place to start, but obviously a bad place to stop. What follows is a list of things we need to consider that might make it wrong.