Spain customs

Immigration will look at and stamp your passport and may ask a couple questions (e.g., how long will you be visiting, where you’re going, where are you staying, etc.). It usually only takes a couple minutes. Once finished with immigration you proceed to baggage claim and get your luggage. For customs, if you have nothing to declare you follow those signs and walk out the terminal. If you do have something to declare, follow those signs.

I think you are asking about immigration/passport control? When you land and you deplane, follow the signs to arrivals (not transfers). You didn't mention which airport/city in Spain, but here is what the procedures are at the Madrid airport:
https://www.aena.es/en/adolfo-suarez-madrid-barajas/airport-services/categories-and-terminals/arrivals-area.html

ARRIVALS AREA

Passport Control

After disembarking, if your flight was from countries outside the Schengen area, you must go through Passport Control.

For more information on the requirements for entering into Spain, see the Ministry of Foreign Affairs, European Union and Cooperation website

Adolfo Suárez Madrid-Barajas Airport has a police-supervised Automatic Border Control (ABC) system that automatically identifies and verifies documents to give passengers fast-track passage through Passport Control. In order to be able to use this system, passengers must be citizens of the European Union, Iceland, Norway or Switzerland, be over 18 years old and have an electronic passport or electronic ID card (DNI-e).

Since the US is not on the list for using e-gates, you will wait in line to have your passport checked/scanned/stamped by a border patrol officer. Most likely, you won't be asked anything but they may ask innocuous questions, such as why you are visiting (business or pleasure), how long you will stay, etc.

After this, you will follow the signs to collect luggage and exit.

Assuming you mean immigration/passport control, I arrived at Barajas on a week day early last month, around 7:30 - 8 AM. Several planes had arrived about the same time. Definitely the slowest passport control processing I have encountered in the past 5 or so years (many foreign ports of entry to compare to). Waited in line for at least 45 minutes, as noted above, no e-gate scans. When I finally reached the official, no questions for me or anyone I observed, just stamp and on your way.

Customs - as noted, follow signs for "Nothing to Declare" (unless, obviously, you are declaring something!)

You can't really miss how the system works. Follow the line and do as told. I hope things move faster for you!

Note that all the above only applies if you are flying directly to Spain from the US, if you change planes elsewhere in the Schengen area, like Paris or Amsterdam, then you will do immigration there. If that is the case, then in Spain, you just walk out of the airport.