To refill the cart, you will put the
juice in the cart.
The other method you are seeing is what is known as direct dripping, or just dripping. With dripping, you don't use a cart -- you simply place a couple of drops of the
juice on to the atomizer bridge and vape until vapor diminishes.
Dripping provides better vapor production, and is less fussy than refilling a whole cart. However, the juice can leak from your atomizer into your battery, causing your battery to get "stuck" in the on position. Sealed, manual batteries are best for dripping.