At the moment, the situation in Europe is changing a bit.
Firstly, if you have the biz class ticket - then you get in. If you have a Y ticket but have a higher FF card - then you get in.
Secondly, there are now companies who operate commercial lounges. These are available by:
1) Walking in on the day and paying the fee.
2) Some Charter carriers provide discounted access but you still pay.
3) A club scheme called Priority Pass, where you pay an annual fee and can then use any of the registered clubs in the world. There are also a partnership between Priority Pass and American Express for Platinum Card acces - but you must still have joined PP and have their card.
Note that if you take in a guest, you pay a fee for them.
SOMETIMES the commer commercial lounge may also host mainline FF pax but this tends to only be at small airports. If you travel a lot in Europe - particularly in Y - then
Priority Pass is well worth having, in my view. Even in the years when I do not use them much, I am glad to know that they are there and it is global.