Why is my iPhone using Wi-Fi now and not LTE like it was before?

I got my first iphone about a month ago. It said that it was using Sprint LTE (supposedly faster connection).
I hooked it up to my MacBook Air and it updated the phone right there. Now it's using my house's Wi-Fi.
My question is… WILL MY PHONE GO BACK TO LTE IF IT CAN'T FIND Wi-Fi SIGNAL?

I'm new with all of the Apple products and syncing them together.

And another thing. I'm downloading music into my laptop. How do I transfer it to my iPhone?

Of course it will. Just turn off the Wi-Fi on your phone and it will return to Sprint Network (4G LTE).