Unfortunatelly EU has no means of cross-border employment yet other than establishing local subsidiary company. It is unlikely to happen if they "employ" a couple of people in various foreign countries. Company setup procedures and employment laws vary by country, salaries may vary by currency. It is just too complex at small scale.
Then the simpler solution is to use intermediary local contractors agency or just directly pay Euro invoices of long term freelancers.
It is legal but e.g. German companies prefer not to keep such contracts longer than 2 years. Why? That is another story...
An example would be Velocity Global - https://velocityglobal.com/
In Canada (at least where I am) you basically earn employee status. I think it's the same in Germany ("Scheinselbststaendigkeit"). So you have to basically ping pong between two places or earn enough to just take a prolonged vacation until they can hire you back coz they like your work ;)