is there a program I can use to clean my macbook and do i need anti virus program?

Sure. It’s built in. Gatekeeper and XProtect, and downloading only from the app store and vendor sites.

Most of the junk folks get is stuff the user intentionally and directly installs. Is that malware? Or adware? Or just a junk app?

Some reading: Effective defenses against malware and other threats - Apple Community

Ditto to both previous Contributors. Worth reading over here What is “Other” storage on a Mac, and how can I clean it out?

None are needed and none should be installed.