I wonder which non-immigrant countries are there? Is Britain an immigrant country?

In theory, all countries can immigrate. The immigrant countries mentioned here should refer to countries that are mainly immigrants and have long-term policies to attract immigrants, such as the United States, Canada, Australia, Australia and New Zealand in North America. European countries have always been immigrant-exporting countries, not what we usually call immigrant countries. Similarly, Britain has always had a policy of skilled immigration and investment immigration, but it is very strict and has very strict restrictions on the number of immigrants every year.