Learn English – What do you call this kind of shoes

I don't know that there is a MOST common one. There are several in common usage, and I suspect different regions of the country use different ones. Where I live, you'd most likely hear:

• Thingy
• Thing-a-ma-bob
• Thing-a-ma-jig
• Doohickey
• Doodad
• Whatchamacalit

I've never heard "thing magic" before. Gadget and widget are used, but I think they are more specific. Gadget would be used for a small machine or toy. It can also be used to describe a collection of little tools, like "I have a whole drawer of kitchen gadgets", that you know the names of, but don't want to list. A widget would usually be used to refer to a piece that is used to build something. Again, I think it is more likely to be used when you don't want to bother naming the thing, or want to talk about pieces of something in general terms.

Good question, and one that most native English speakers won't even get correct.

The icon you circled shows the strength or quality of your wifi signal as measured in bars.

Bars doesn't really mean much, though: it's not a regulated unit of measurement like grams or decibels. Different systems use different amounts of bars, so what's important is how many bars you have out of how many bars there are total.

You could say "My WiFi signal is very weak: only 2 out of 5 bars."

And were it a better signal...

"My Wifi signal is okay: 3 out of 4 bars."

"My Wifi signal is really strong: full bars!"