This is called the "lump of labour fallacy". There aren't just a fixed number of jobs to be filled by the people who live in a country. As more people arrive, those people need to pay for services such as rent, groceries, cars etc. etc. creating more work and more jobs. The fact is that the majority of immigrants in the UK are young, and therefore contribute a disproportionate amount of tax relative to what they take out from government services.

It's not all roses - UK companies, such as fruit farms, are able to treat low-skilled immigrant workers worse than long-term residents. But there are also plenty of high-skilled immigrants, so to pretend that it's about countries being "sold out to stock markets" is naive. Just wait to see what happens to the NHS as foreign workers leave over the next few months.