I had a look at these antique bikes today, and found out that there is still a number ("a number" is very imprecise, but definitely tens, if not hundreds) of people who rides them.

So this intrigued me, and what I don't quite understand is: do these have some sort of freewheel mechanism in the crankset itself? What is the stoping-starting technique? And how exactly are you supposed to turn? I guess you can do that by changing the baricenter, but you sure can't make sharp turns that way, can you.

In any case, a very interesting kind of bicycle. Hey, maybe it fits right in the singlespeed movement

They are the true fixed-wheel bike and a lot of fun to ride. Google around and you'll find the answers to your other questions...

Starting and Stopping is done on the fly. To start, you have to get on the left side and start pushing the bike forward, then put you left foot on a peg above the rear wheel and swing your right leg up over the seat, all in one smooth motion. Then you get your feet on the pedals and go. Dismounting is done by slowing and sliding your left foot back to the peg as you bring your right leg around to the ground.
The front wheel steers like any other bike. Most bikes had a rod operated brake that rubbed against the tire tread. The brake pad looked like a steel shoehorn. The crank was fixed to the wheel's axle, so you could slow down by pedal resistance, too.

Check these guys out:

https://www.thewheelmen.org/

True antique ordinaries re getting pretty rare and pricey. There are repros of varying quality available.