DROPPED BALL

I watched a game recently where an incident occurred that neither side benefited from, nor lost from; it was a stray ball on the pitch that had left the reliable hands of a ball boy chosen from the junior team member of the home club. It is quite rightly regarded as an honour for young lads to be inside the barrier as ball boys, so near to their adult heroes!

The referee blew his whistle, play stopped, the stray ball was removed and the ref negotiated a "re-start" from where the ball was played as he called the game to a halt. It could have been at a place where one team gained an significant advantage, but usually the ref waits for a neutral bit of play to blow his whistle.

When there is no other possible restart (that's what they are called-a free kick, throw in, etc) the dropped ball is the solution, offering no particular advantage to either side.
There might have been a serious injury, interference from an outside agent (a dog running on the pitch), or the ball getting dangerously trapped under a player who is on the ground.....there are lots more incidents of course!

So the ref could could just drop the ball once he has made all the players aware of his decision. There is no ruling that says there must be one player from each side. Actually, that is what normally happens and this often brings a confrontation that can spark off another free kick! Depends if you put your clogger into the "duel".
https://www.youtube.com/watch?v=0umwJvtQXC8
Often these days the players decide who deserves the possession, such as a pass to the opposition goalie, and everybody claps their sportsmanship!

Originally there was a "throw up" where I guess heading was the next touch and in 1905, like Aussie Rules, there was a "bounce up". In 1914 the dropped ball came into being, where the ball must touch the ground. The ball should be dropped from about the ref's waist height not bounced so the next contact is often close to the ground, around the shins.

Dropped balls inside the goal area were banned in 1984 to avoid too much "competition" and taken by the ref outside the six yard box.

A goal can be scored from a dropped ball.
In January 1999, the Preston NE goalie kicked the ball out to allow a Wrexham player, who had a head injury, some treatment.
The dropped ball took place about 30  yards from the Preston goal.
The Preston players stepped aside to allow the Wrexham player, Jeff Whitley, to pass the ball back to the Preston keeper. Whitley's superb half volley cleared the Preston goalies' head and into the net.
The ref disallowed the goal and awarded Preston an indirect free-kick for "unsporting behaviour". I suggest Whitley didn't mean to score!

The dropped ball is the only restart where a player may play the ball twice before it is played by any one else.