Probably not possible with just obs unless someones writes an adb capture plugin (I couldn't find one). I ended up getting the program Vysor and just used regular old window capture on that. Definitely isn't the best solution, it doesn't even scale the window to the resolution of my phone, so I have to adjust that everytime my phone switches between portrait and landscape. Also never tested the sound capture through that program as I already had a cable I used for NDS for the same purpose on my desk.
once upon a time I saw a work-around way for 3DS recording without a capture card enabled 3DS which basically plugged an iPhone into a computer USB then set the phone as a camera device within quicktime. Granted this was iPhone specific so idk if this works for all phones
EDIT: Clarification: This is capturing using the phone as a camera, not capturing the phone's screen itself (which after rereading was what I think you meant)
No I meant using the camera on the phone (Android in my case)
Oh then plugging in the phone via USB might be possible to set Phone as the camera device
I've tried a program called Droidcam before for that kinda stuff. It's not the best (quality isn't the best. I've noticed delay and stutter over wifi) but it works and it's free so ¯_(ツ)_/¯