That's the answer: both are SMS/MMS apps. SMS and MMS messages are always sent via mobile networks. Even if you're in a WiFi 'bubble'.
MMS is by far the slowest and most expensive m.o. to send images and videos. It can also send only very limited size files = lo-res.
If you want to send (many) hi-res images and videos you will need to use the "IM protocol', Instant Messaging. Good IM apps are e.g. (free) Telegram (for text messages, hundreds of photos, or videos of up to 1 GB), and (also free) Viber (includes 'VoIP' [internet audio calling], and video calling).
Both the sender and the receiver need to have the appropriate app installed.