Lots of questions? Correct.
Some things to consider that were not asked.
From the US to Europe...
This hint has diminished in value, depending on the airline. Book the flight from your origination airport to your destination airport. This way, your luggage allowance is (or is supposed to be), whatever the international allowance is, vs domestic allowance.
If you change planes in Europe, any duty free liquor you bought in a US airport must be packaged in a manner acceptable to EU standards, or it will be confiscated.
If you choose to stopover in a European city and reboard a plane to your final destination in a day or three, there are new rules for your luggage, based on weight, not pieces, and you probably won't like them.