During the course of the game you will unlock items for a total of 200 items per game. There is a total of 500 items in the game so there is some RNG involved with getting blocks that will complete the set.
Once you have four of the similar items you will need to group them together. You can either hold all four at the same time or place them next to items that are already on the floor. They must be touching but it doesn't matter if they are in a line, a 2x2 box or any other shape.
The Items you need for this are four VERY similar looking birds, only their beak is different.
The best time to match these up is during the untimed
"Stock Take" phase of gameplay.
Also you can revisit warehouses that you completed.