When blocking communities, they will not show up in your "all" feed anymore. This makes browsing Lemmy nicer especially now that you might not subscribed to a lot of communities.
In the browser: In you Lemmy instance, click on your username (right upper corner), "Settings" and open the tab "Blocks" and type in the community you would like to block. Here you can also unblock communities
On Jerboa (Android): Open the community you would like to block, click on the 3 dotted menu (right upper corner) and click "Block Community". Note: you can not unblock on Jerboa. Use the browser (Settings -> Blocks) to unblock.
It depends on the kind of immigrant. You have students, high educated workforce, people that flee from war/not safe to stay country and people that just want a (economic) better life.
I think too much of any immigration can cause maybe an issue that the majority of people are new and that the culture (how do we interact with each other, what is acceptable behavior etc) has not settled.