Safety briefing: DON'T DIE!
Relative work is the most straightforward evolution after graduating. Students are taught the "boxman" position as their first stable freefall position. From there, it's easy to turn, move forward or backward, and even slide sideways. Relative work is just the art of performing these moves in coordination with other skydivers to form pwd figures while in freefall.
Everything is in the coordination and the accuracy of each move. Turning in the air without moving is not that easy! Too much pressure on the legs and you will move forward. Not enough and you'll move backwards. To make things a bit more difficult, the objective is to actually form the figures by taking the grips. Not obvious when you are already using your hands and your arms to keep your balance in the air.
These are the most common disciplines in competitions. 4 ways are the fastest because the formations are much less complex than the bigger ones. However, nowadays, some 8 way teams are pretty fast too. 16 way competitions are much less usual, first because it's much easier to train a team of 4 than a team of 16, then because 16 ways require a much bigger aircraft.
There is no real definition of big formations. Depending on your level, 10 can be already big. The biggest formations in Indonesia is 24. In large drop zones like Zephyr Hills, formations of 100 and more are far from unusual. The 282 way world record was completed above Ubon Ratchathani, Thailand, on December 16, 1999.