To quickly search for a product, first you have to activate the "item search" module on the Back Office:
Go to your Back office
Click on Configuration
Click on Your modules
Click on Order taking settings
Activate the "Item search" module by clicking the red button on the left and turning it in green
Go to the Tiller app
Click on synchronise on the top right to refresh it
When you open a new order, at the top of your categories there will be the search button for quick searching for a product (red circle).