Touch it, walk on it, or shoot it. The majority of doors on Stroggos open as you approach them. If one doesn't open automatically, seek a button, floorplate, or key.
Most secret doors open when shot.
Others open by activating floorplates, buttons, or levers. Specific symbols indicate the exit to a given area. The Strogg marked off areas to indicate a one way passage. Once you leave a unit complex you cannot return. The Strogg have concealed military supplies in different locations within the city.