iPhone data usage when cellular data is on

if I'm connected to a WiFi network, does my iPhone still use Cellular Data?

No

If I'm going out and I'm outside of my WiFi range (disconnected to my WiFi), iPhone will automatically use my Cellular Data in this case right?

Yes

And when I came back (or connected to another WiFi network), iPhone will automatically use the WiFi for data connection and not "Cellular Data" right?

Yes

Things are a little more trickier if WiFi Assist is enabled on your iPhone. In this case, cellular data will be used if the WiFi signal is poor.

Home Screen > Settings > Cellular

Scroll way down to see WiFi Assist

Don't worry- iPhone will not use Cellular data if a known Wi-Fi network is within range. Whenever you come into range of a trusted Wi-Fi network, it'll automatically use that network instead if cellular data.

The only exception is if "Wi-Fi Assist" is turned on- this allows iPhone to use cellular data to supplement its connection to the Internet if the Wi-Fi is exceptionally poor.

fsoc wrote:

My question is, (1) if I'm connected to a WiFi network, does my iPhone still use Cellular Data?

No, with a couple of exceptions. If you send MMS (picture) messages to non-iPhones MMS will use cellular data. And Visual Voicemail always uses cellular data. The reason: Both of these features go through your carrier, and your carrier network doesn’t “know” about your Wi-Fi network.

Other than these exceptions, Bob Timmons is correct.