History of the Shipyards
Republic Sienar Systems were dissolved by the Republic during the Clone Wars. After the end of the war the remnants were sold off with Kuat Drive Yards buying up most of the assets and soon after bringing Raith Sienar back to resume work on the TIE/Ln. This was the start of Sienar Fleet Systems.
The Republic soon after received the T-65 X-Wing from Incom Corporation to replace the aging Z-95 and ARC-170 models from the Clone Wars. This steered the Republic away from the new TIE series and towards Incom. KDY was about to pull the plug on SFS, but this was also the time that the Unity was looking for a new fighter design. Hoping to save his company from a second fall, Raith Sienar took the Sith Fighter that was rebuilt during the Clone Wars and used it as the bed for a new TIE Fighter that could be used in the Unity's current ships without needing to retrofit their hangers. The Sith Knights bought the design and contracted Seinar to build them.
Kuat Drive Yards accepted to keep Seinar Fleet Systems to show support for the Unity who usually only used the Star Forge for building their fleets, KDY then increased Sienar's budget to supply the Unity with any fighter needs. This left KDY free to become the primary contractor for the Republic Navy's warships, working together with other companies from time to time.
CEC became very isolated in this time. While still one of the largest civilian shipbuilders, they no longer held the protégé of a warship builder. They began collaborating with other shipyards to stay in the game but with Mandal Motors, Mandal Hypernautics and Sienar Fleet Systems taking up the Unity's needs for non-Star Forged warships; they were forced to work solely with the Republic and even work together with Kuat Drive Yards for most of their ships. They however were happy to work on a personal transport for the Lord-Phantom.
A new ship building company was born in with the success of the Defender and Defender II-class Frigates. Designed by Lord-Commander Drake during the Clone Wars and built with secret shipyards hidden in the Malachor V asteroid field to construct a fleet independent from the Republic to safely break away at the end of the Clone Wars. After the Unity was stabilized Drake turned these shipyards into a company called Aegis Industries, a ship building and designing company with a love of the old Sith style. Aegis soon grew to become the leading non-Star Forged ship builder in the Unity, able to build all of the vessel types that were usually built by the Star Forge albeit at a much slower rate and higher cost.
So this is how it's separated. There are smaller companies but most of those are just subsidiaries of these major ones or too small to matter or just don't have a ship I'm using in the mod.
Kuat Drive Yards
Mon Calamari Shipyards
Seinar Fleet Systems
Corellian Engineering Corporation
PS: All future TIE models will be for the Knights. Only the TIE Fighter and Bomber will be used by the Republic.