How do I scan my MacBook Air for malware or viruses using built-in tools?

the build in security is on all the time and not something you manually has to start scanning

if you wish to scan something then you can download the free version of malwarebytes and etrecheck

and manually scan using them every now and then

1 - Windows-like viruses that self-replicate and affect macOS don’t exist because of the underlying UNIX Foundation and permission limitations.

The macOS operating system resides in a sealed and read-only volume that can’t be opened by users or third-party applications

2 - Mac app security enhancements are sufficient to protect computers from malware.

Adware and malware can affect computers and are often downloaded from “shady” websites or developers who inject them into downloads.

Source: User tip from @Kurt Lang

What is malware? - Apple Community

I am concerned I may have downloaded a document or application that should not be trusted.

Did you run an installer and provide your password? if not, there is nothing to be concerned about. Reading an email, downloading a file, visiting a webpage, and any other passive contact with malware won’t infect a Mac.