Plenty of YT content out there that's just cellphone-made.
One of the key things is in the editing software (and the patience and willingness to spend time with same).
One critical item can be a tripod or similar way to hold the phone securely.
Using all voice-over audio can be simpler sometiems than relying on the phone's audio--this can be easy or difficult based on the video editing softare used (and more so with the freeware versions).
And, Kathy Millet does make it look easy. (As does Luke Towan.) Some of that is in the patience to film all the "takes" it takes to make a video.
It's a skill, like anything else.