They are put into collections based on the pictures at hand. They can be unlocked via the Gift Machine on all games or playing a song multiple times on Just Dance 2020 and Just Dance 2021.
In Just Dance 2018, the stickers are based on all kinds of elements. All You Gotta Do (Is Just Dance) will be playable if all 10 CD-styled stickers are collected.
In Just Dance 2020, the stickers are based on all the Just Dance games in the main series. They are also used to unlock Aliases depending on which page has been fully collected. Dancing to a song in the main track list three times unlocks stickers as well.
In Just Dance 2021, the entire collection of albums are themed on the background of Without Me. A song in the main track list only requires to be danced to twice in order to unlock a sticker now.