Generally this part is a command module in order to provide "control" for the craft - control is only given by command modules, and without it you cannot make the craft do anything (it ignores your commands and continues doing exactly what it was doing previously). This part can be changed with the Root tool. When building any vehicle, you will first have to select a root part (note that not all parts are allowed to be root parts). If you are reading through this guide with only the demo version, you will not have the SPH available to you, nor many other features offered within the game. The basic mechanics of building vehicles are the same for each building, and you have the same parts available for both but due to the orientation you use, they tend towards differing styles of vehicles (VAB for Rockets, SPH for Spaceplanes). The Vehicle Assembly Building (VAB) is where you can build vertical vehicles using radial symmetry, and the Space Plane Hanger (SPH) is used to build horizontal vehicles using bilateral symmetry. In sandbox, only the date/time and time warp controls are displayed. In science mode, your current science is shown there. When in career mode, you current funds, reputation, and science are displayed in the top center bar. Flag Pole - in front of the Astronaut Complex, this is where you can change your default flag.Mission Control - this is where you can view and accept contracts (not available in Science or Sandbox modes).Research and Development - this is where you can research new parts in exchange for science (not available in Sandbox mode).Astronaut Complex - this is where you can hire new kerbals into your space program, and see the status of your astronauts.Tracking Station - this is where you can observe orbits and access previously launched craft.You can also launch here without going through the SPH.
Runway - this is where vehicles from the SPH are launched.You can also launch here without going through the VAB. Launch Pad - this is where vehicles from the VAB are launched.Spaceplane Hangar (SPH) - this is where you build horizontal vehicles using bilateral symmetry ( spaceplanes).
Additionally, you are also able to Check for Updates to your game.
The game's main menu presents you with the options to Start Game, adjust the in-game settings, visit the KSP Community website, visit Curseforge - a community driven mod site (replaces SpacePort), view the game's credits or quit and return to the desktop.