Shift is exit the vehicle, and enter/return is enter the vehicle.
Hold down the left mouse button to fire.
And on the right is a drop down collapsible menu where you can open it and it has items you can use to help you. Bombs, mines, towers, shields, and blocks. Click on them and place them where you need them.
Right mouse button is to end the item placing.
https://jamestown.itch.io/star-cao
Full version here