How Can I Check for Malware or Trojans on macOS Big Sur?

I’m using a MacBook Pro running macOS Big Sur, which I downloaded from a third-party site. Everything was fine until recently, when I noticed the Photos app (along with a few others) opening on their own, even though I’m pretty sure I didn’t touch them. Now, I’m starting to feel a bit concerned that there could be some kind of malware or trojan lurking around on my system.