Sim and eSim cards

Just an FYI, it is a SIM card (a physical chip) or an eSIM data plan (an embedded chip). I prefer eSIM plans but a lot depends on your phone, whether it is locked or unlocked, your cell provider and so on.

More info would help you get more tailored advice, and some of your questions are dependent on that. For example, who is your cell phone provider? Different providers have different international plans and you may not need to buy anything.

What kind of phone do you have? How long is your trip? and what do you need it for; i.e., data only (GPS, web searching, etc) or data + phone use?

You'll find that the major carriers in Spain are competitive. I pay Vodafone 15euro for 100gigs to be used in 28 days. It includes 600-800 minutes of talk time, but I never use that. Interesting note: many plans there do not include outgoing SMS texts. (Use WhatsApp, like everyone else...)

You can get Spanish/European SIMs in North America, but in country is cheaper. If not the airport, a phone store close to your hotel on arrival day. You'll need your passport. Because most pre-paid plans expire after 28 days you would not want to set it up more than a couple days before a 24-day trip.

I have my US number on eSIM and the Spanish SIM is physical. Verify you have a dual-SIM phone, and it isn't locked by your home carrier.

My phone is unlocked. I have Verizon and the international fee is $10/day. 24 day trip.