If you’re multilingual, what language do you “think” in?

I thought of this when I was in college and one of my best friends was primarily Spanish speaking but he was also proficient in English, while I sadly only really speak English. We were hanging out one night and he answered his phone to speak to someone and he switched over to Spanish to talk with them, then switched right back to English afterwards with me. Since his primary language is Spanish, I had the random thought to ask him which language he thinks in. The question kinda threw him off for a second because he’d never really thought about it before. Ultimately, he said he primarily thinks in Spanish. But then I started thinking about if he switches to thinking in English when he switches to speaking so, or if he stayed thinking in Spanish even if he was speaking English. I think I gave him a headache with all the inquiries out of the blue.

To extend my thoughts now: I still wonder this question occasionally, but even more so when someone knows more than two languages. I wish I thought of it in high school when I had a friend that knew five different languages; that would have been a fun brain to pick.