How do Oysters/Clams create pearls

Do they just collect bits of sand or something?

BQ: how long do does it take to make the pearl

It takes at least 5 years for even a small pearl and the big ones take 20 plus years to form. Most contain a grain of sand at their core. The sand grain gets trapped between the mantle, the shell making organ, and the shell. The pearl is made of the same material that makes the inside of the oyster shell all white, smooth and shiny. It is just layered onto the grain of sand rather than layered onto the inside of the shell surface. The best pearl making mollusks are the ones that make a thick shell very quickly.

Yes, basically. Some piece of sand or other irritant enters the shell where it then starts to irritate the oyster. The oyster then secretes a liquid which coats the grain of sand to keep it from further irritating it. The liquid builds up and hardens until it forms a pearl. When pearls are "man-made", they usually manually insert grains of sand into a farm of oysters to cultivate them.

If a grain of sand gets inside, the creature makes the pearly covering so that it is not scratchy. I don't know how long it takes.
This is how cultured pearls are made: people insert a foreign body into the creature to make sure it makes it. So they are real pearls but the start of it was on purpose not accidental.