Step 1: Click on Checkout, then click on Behaviour and Edit Behaviour.
Step 2: Click on Behavior tab. Enable the Fullscreen select inputs on mobile version button, if you would like to enable the fullscreen select inputs on the mobile version of your checkout.
Step 3: Once you are done, don’t forget to click on Save changes
You can enable fullscreen select inputs on mobile version of the checkout at your checkout by clicking on the Checkout tab in the menu of your Admin panel of the selected store.