Does it matter if you're using a VPN do hide your ip on an Android phone when you use apps like youtube, facebook, or third party open source apps like NewPipe (youtube) or "Tinfoil for Twitter" or "Tinfoil for Facebook"? Will WebRTC give away my ip anyway, or is that just for browsers?
WebRTC only returns private IP Address of a cient and not public IP Address because WebRTC client has no idea what your public IP Address is if you are behind NAT and this is what WebRTC client wants to find out.
If you are using VPN, your request to STUN server - who is responsible for discovering your public IP Address and behind what type of NAT you are sitting at, sees your request coming from VPN.
Any app which is implemented with webRTC API is a webRTC client and yes they can leak your private IP Address. WebRTC is required by those apps who want to support P2P-VOIP and video calling so there are only handful of apps.
YouTube does not use webRTC. Facebook messenger and hangout use webRTC for voice calling so they can discover your private IP Address. Browsers were the first which came with integrated support for webRTC API and this is the list of websites that are using webrtc.
If you are wondering what What's app use for voice and video calls, it uses PJSIP libraries.
If you are using VPN and you find that your real IP Address is leaking, then that's the fault of your VPN client. It is not tunneling your traffic properly. STUN server should only see the IP Address of your VPN server. If this is not happening, it means that webRTC client is making STUN request outside of the VPN tunnel as shown in figure.