ELI5: You can pretty easily tell if something is a book right? So you are looking for something to read. Pick it up. Is it a book? No. Toss it. Yes? Read it.

Search engines do the same with everything they process. Malware can't be embedded in a webpage, its a seperate executable downloaded by the page. So anytime the crawler reads something "is it a webpage?" No, toss it. Yes, process it, then find everything it links to, repeat.