I've got an iPhone se, how do I set it up so it picks up on mobile Wi-Fi and uses that automatically rather than my 4g mobile

I've got an iPhone se, how do I set it up so it picks up on mobile Wi-Fi and uses that automatically rather than my 4g mobile

Connect to a Wi-Fi network. That's it. All iPhones prioritise Wi-Fi connections over data connections.

With my phones I register mobile hot spots with Comcast/Xfinity, my home cable provider I had to download the Xfinti Hotspot app. You could turn off mobile data, It seem random to me if it goes on Wi-Fi hotspot and a lot of businesses that offer free Wi-Fi insist on you signing in every use.