Locked - no. In the unlocked one - yes, but even here it is not so simple. Keeping the microphone on all the time will drain the battery faster, but it will be difficult for the average user to notice.
It's possible. As mentioned above, technically it works like this: listening to the surrounding world is constantly in search of the necessary commands. Those same voice-activated TVs connected to the Internet can eavesdrop on you.
Some enthusiasts have experimented with popular instant messengers. Some of them, holding the phone in their hands for a certain time, said that they had run out of food for the cat, and they urgently needed to buy it. As a result, an advertisement for cat food began to appear in the application. Others say that it is necessary to buy new furniture, hence the advertising of furniture stores.
Most likely this wiretapping is necessary not to listen to your gossip, but just for marketing.
Yes, they can. After all, for the work of the voice discovery, you must always listen, as was correctly noted. Now I don't remember the cases when the giants were caught in this, but who knows how these applications work with the microphone, the code is still closed.
Perhaps this function was made for listening. Perhaps (yes, in general, it is), all the new items are done to collect our data. The time is like that.