Thanks to something called the Schengen Agreement, most of Europe is essentially like one big country for travel purposes. Completely open borders.
Of course, it’s one thing to know that theoretically, and it’s another to be sitting on a bus and realize that you crossed over into an all new country and didn’t even realize. Because that’s exactly what happened to me recently — I was riding on a bus from Porto to Madrid (a punishingly long nine hour bus ride), and I was expecting some kind of cursory border check. Anything at all. But nope.
It’s super bizarre to be travelling to another country and crossing the border is essentially like traveling to another province in Canada, or to another state in the US. No passport check, nothing. I’m sure there was some kind of “welcome to Spain” sign, but I guess I missed it.