Let me begin this by quoting a well-known aphorism: Two Jews, three opinions.
It is an exaggeration to say that there are as many answers to this question as there are observant Jews. It is now 1:24 EST, which means it is well past sundown in Europe and further east, so we might get some informed opinions soon.
But it has been a minor amusement over the years to discover how various sects interpret these rules. At least one observant Jew I know in Israel had his lights on a timer that must have turned them off on Friday night and turned them on at 3 on a Saturday afternoon in April 40 years ago. My (non-Jewish) DIL was working at Beth Israel Hospital in NYC some years ago and the first time she discovered the Shabbos elevator that went up and down endlessly, stopping at every floor she was astounded. There were other elevators that operated normally. Food will not be cooked, but it might be left on a very low light all night and the next day, just to keep it warm. Telephones will not be answered. Nowadays you can uplug them all and turn off your cell phone.
But in my mind, the most amusing thing concerns the eruv. There is a prohibition against “carrying” on Sabbath. Amusingly, in some languages, e.g. French, the word to carry and to wear are identical, but I don’t think it extends to not wearing clothes by anybody. One result is that when you walk to Shul (synagogue) on Sabbath, your pockets are empty. So what do you do about a key? Some people, I imagine, just leave their houses unlocked. Some just carry keys. In Brooklyn, I have seen doors with combination locks on them. I guess turning the dials (or pushing the buttons) doesn’t count as work. But carrying is allowed inside your dwelling.
For some, this opens an out. Suppose you declare that some large area is your dwelling. But it has to be delimited in some way. This is called an eruv. The usual way is to create a conceptual boundary of overhead wires. In my town, a suburb of Montreal, some people got together and designated collection of wires as an eruv and blessed them. (I might mention that, although no new wires had been strung, some other people in the town took strong objection to this.) At any rate, it is now legitimate to carry things in this town so long as you stay inside the eruv.
Some years ago I was walking down Flatbush Ave. in Brooklyn with my wife and she spotted a thin wire hanging far above the sidewalk and asked me what it was for. You have to understand that NYC does not generally permit overhead wires. Back when there was a trolleycar on 42nd St., it got its current from a wire through a slot in the street. So I guessed it was an eruv. I asked an orthodox Jew of my acquaintance and he agreed that’s what it was. He also told that there is an organization that strings this. And that there is another organization that goes around taking them down because the latter think you can’t fool God.
I could go on and on with these stories, but I stop here. Warning: I probably got some details wrong because I am totally non-practicing.