Each pre-built template has different column structure for checkout fields. You can modify the columns using Aero Checkouts CSS Ready Classes provided in the customize Classes section of Checkout Form.
Open the customizer of your selected checkout template > Checkout form> Classes and then edit the ready classes as showing below
2) Single / Multi Columns Classes
You can update the classes using this pre-built column classes to get 1 / 2 / 3 Columns for fields.
wfacp-col-full – To create full width field
wfacp-col-left-half – To create two columns structure and set field left side
wfacp-col-right-half – To create two columns structure and set field right side
wfacp-col-left-third – To create three columns structure and set field left side
wfacp-col-middle-third – To create three columns structure and set field middle
wfacp-col-right-third – To create three columns structure and set field right side
wfacp-col-two-third – To create two third columns structure
wfacp-col-clearfix – To create field from new line use clearfix
See the preview below when you apply these classes