What effect has migration had on modern Europe

For countries / cities that are losing people there will be fewer people to housed and fed.

and reduces pressure or competition on jobs and resources.

For countries/cities that are gaining people there will be more competition for skilled jobs (if the migrants are young and skilled) or high labour jobs if the migrants are not very skilled.

This means that the population density will increase and more people will need to be fed and housed. This then creates a diversity in the communities which can have both negative and positive impact.