Mobile Service is your cellular phone calls and voice mail from your cellular service provider which use cellular protocols such as 3G, 4G LTE, 5G UW to communicate via cell towers. Mobile Data is packet data communications via networking protocols such as TCP/IP, UDP, RTP. RTMP which allow applications to communicate via the internet.

When you see the name of your provider AT&T, Verizon, etc in the top left or your iPhone, your cellular phone calls are being routed to and from your phone via the cellular protocols of your Mobile Service via their cell towers. When you see Verizon Wi-Fi or AT&T Wi-Fi, your smart phone is using Mobile Data via the internet to send the phone calls and voice mails to and from your service provider.