How does coal get into the trees?

Answer 1

You are speaking of charcoal. Charcoal isn't coal, it's a partially burned wood. Coal is a burnable fossilized fuel, and charcoal is from incomplete burning.

They burn wood in a controlled environment to keep some of the combustible elements in it. Then they compress the charcoal into briquettes for home use. It's still just wood that is burning; it's not real coal. Charcoal looks similar to coal, that's the reason it is called charcoal.

Coal is also formed from trees, but takes many many years to form: Millions of years.
For coal to form, a large number of trees need to be buried so that they cannot rot quickly as they would if exposed to the air. If they are deeply buried, the pressure and heat of the earth slowly transform them into coal. The longer the time period and the higher the pressure, the better the coal will be. Good coals are almost pure carbon.