#82 TIPS: Filler bricks
Filler bricks are used in large scale MOCs to build up the landscape or ship that needs bricks to support the structure. These bricks won't actually be seen, so builders often use any colour and size they can get their hand on. They won't be seen in the final thing as they are covered by the right colour bricks and other details. This brings down the price of the MOC, as you can use any pieces you have in your loose collection.
If you are unsure about filler bricks or just want see them in action, check out this video -
You can flick to the timestamp of 2:20 to see the where the filler brick is used. The builder also uses Duplo bricks to build up to landscape as these are cheap and big so fill up the space well.