In many European countries the presence of unions isn't optional, it's compulsory above a company size threshold for example. It doesn't mean employees have to be part of it, but it has to be present.

At least where I live (Norway) your employer doesn't have a say in whether you're organized or not, and wouldn't necessarily know if you are – but you'd typically have an elected representative in companies of a certain size or with a certain number of employees.

Most unions I know of operate on a country-level.

Engineers in the UK could join Prospect, although there are also alternatives for more specific fields.

They have an advice line, but don't generally help non-members.

https://prospect.org.uk/article/getting-help/

UTAW looks like a new British union for tech workers, set up by Google and Microsoft employees.

https://utaw.tech/about