[Alneyan]

You cannot launch weapon platforms, these can only be present on planets. If you want to transport these platforms to another planet, you will have to use a transport and the "Transfer Cargo"/"Load Cargo" function.

You can use resupply depots to give supplies to your ships if they are over a planet, or Stellar Collectors (found under the Stellar Harnessing field) which generate supplies in deep space. Or you may want to build a few ships dedicated to storing supplies if you are going to the offensive. Later on, you may discover other components to generate supplies, such as Emergency Supplies Pods or the Quantum Reactor, which basically solves this problem.