[AC Question]

I'm new to android and have a Moto E. I had a few issues connecting to WiFi at work so am familiar with the '!' that appears against the WiFi symbol when no internet connection is available (that issue at work has been solved).
Today, at home, I tried the YouTube app for the first time just to test it out and it worked fine. After 10 minutes or so though, I received an alert telling me that I had reached 75% of my mobile data allowance, which surprised me because yesterday I had hardly used any. I checked the usage stats and it was indeed YouTube that had eaten up all the data, however, according to my phone I was connected to my home WiFi. As I hadn't previously used the app, it had to be this session that had gobbled the data.
I turned off mobile data to see if that proved it, and it seemed to, because when I turned it off, the '!' appeared against the WiFi symbol immediately, telling me I had no internet connection. I tried a few other apps and none of those worked either.
I re-started my phone and that solved it - I now had access to everything again, obviously via WiFi (because data was switched off).
My point here though is surely if WiFi isn't available I would have expected the phone to warn me that it was using data, not wait until I had shut data off before telling me that there was no alternative connection available.
Mobile data is now permanently switched off!

[belodion]

Welcome.

Yes, normally if you have both wifi and mobile data switched on, the phone will use wifi, whether it's running Android or any other system. I'm not sure what happened in your case. I'll refer this to others who may know.

[dpham00]

The phone usually will automatically switch to mobile data without any warning messages when wifi isn't available or there is a problem with wifi . This is typical for all the phones I used.

If you don't want to use mobile data, use Tasker or similar to disable mobile data as soon as you get home, and enable it again when you leave.

[Tim1954]

There is a setting called "smart network switch" that will use network when the wifi is a bit slow or patchy. You will find that in your wifi settings. Exclamation mark next to wifi icon would mean you have no connection?