Probably the most common way to handle that sort of thing is to put the different POV into different chapters. Like, most of your chapters might be protagonist 1st person POV, but occasionally you have a side scene from a different POV and with different characters.

If you set up the scenes carefully, you don't even have to label them. It should be obvious to the reader when a chapter comes along in a different POV.

This is analogous to a cutscene in a video game. Most video games are essentially first-person, but sometimes you get a movie from the bad guys' point of view. Happens in movies and TV shows a lot, as well. It sets up dramatic irony when the main protagonist has no idea what the others are up to, but the reader does.