The primary factor is that 120mm is not the actual focal length of the lens in the phone, that will be a considerably smaller focal length that happens to give the same field of view as a 120mm lens on a full frame camera (i.e. the same sensor size as a 35mm film frame). The phone uses a much smaller sensor than this hence the smaller focal length for the same FoV.
Another thing that helps is using a telephoto lens design - including this lets you create a lens noticeably shorter than its focal length.
When the lens would still make an overly wide phone despite all this that’s where the periscope lens design as others have already mentioned comes in.
The primary factor is that 120mm is not the actual focal length of the lens in the phone, that will be a considerably smaller focal length that happens to give the same field of view as a 120mm lens on a full frame camera (i.e. the same sensor size as a 35mm film frame). The phone uses a much smaller sensor than this hence the smaller focal length for the same FoV.
Another thing that helps is using a telephoto lens design - including this lets you create a lens noticeably shorter than its focal length.
When the lens would still make an overly wide phone despite all this that’s where the periscope lens design as others have already mentioned comes in.