Quoted:
The design team at Incom stole the specs for the T-65 Space Superiority Star Fighter (X-Wing) and defected to the Rebel Alliance. It was originally intended as a design for the Empire. Sienar Fleet Systems was the runner up with the TIE/Ln Star Fighter due to the loss of prestige by Incom. The T-65 was equipped with a hyperdrive engine from the start and with shields too. It was a successor to the Clone War era Z-95 Star Fighter (Headhunter) used in Republic Service. Mara Jade piloted a modified Headhunter as a Imperial Hand.

The original goal of the T-65 was to be a deep strike capable star fighter for long range patrols against Clone War holdouts. But with the mop up of those forces, the Imperial Navy switched to anti-piracy and interdiction duties. The TIE/Ln served a better role since it was used in conjunction with corvettes, frigates, and destroyers. The need of equipping hyperdrive on a TIE/Ln was seen as a waste of resources due to doctrine, not technology.

The TIE Advance X1 (Vader's TIE) was equipped with hyperdrive.

Anyways.....

Rebel Tactics was hit and run. They jump in system or a known travel route and intercept a Imperial convoy. The Y-Wing, another Clone War era fighter bomber was good at this against lightly armed transport ships. But when TIE/Ln Fighters started popping up everywhere (like the UK had Spitfires and Hurricanes on catapults), the Y-Wings started getting their asses handed to them. So the Rebels developed the A-Wing. A lightning fast interceptor meant to harass convoys.

The Empire countered them with TIE Interceptors. They were faster and more agile than the TIE/Ln. At first, they weren't equipped.with hyperdrives since they too were assigned to convoy ships and such.

Incom stayed as an Imperial contractor, producing the I-7 Howlrunner long after Palpatine died, but the Imperial Government stayed in power. It never outright collapsed.

Hyperspace was a separate dimension of existence. It worked like a wormhole. It wasn't traditional FTL like Warp Speed in Star Trek. In Star Wars, it was known and mapped safe routes. Because the gravity well of a large body could affect travel.

Here is a map of the major hyperspace lanes.

https://starwarsphilosophy.files.wordpress.com/2015/05/galaxy-map-unedited.jpg

In the original EU. It was stated that pirates going beyond the founding of the Republic would tow large asteroids into a known lane to have ships drop out of hyperspace when the safety system detected a gravity well. They essentially road blocked the hyperspace lane. The Empire went so far as to build gravity well projectors and installed them on frigates. The original EU stated that you couldn't jump to hyperspace within the gravity well of a planet or system. You had to get to the outer edges to jump. So the Imperial Navy would park an Immobilizer 418 Cruiser on the edge of a battle space or the edge of system and turn on the gravity well projectors and keep ships from jump. They'd also park them by known hyperspace lanes and pull ships out of hyperspace for boarding.

Hyperspace was literally a mapped out network of safe travel lanes like the autobahn and surface streets.

The Unknown Regions were called that because no one has mapped it out. The deep core was dangerous too since it had a tight collection of gravity wells from stars and systems being bunched up. Also, the planet Kessel was a known danger. It was deep in the Maw, an area infested with Blackholes that were put there to keep an especially powerful force sensitive captive by the same species that created the Corellian System with CenterPoint Station.

Han Solo's Kessel run was famous because he took a shorter route and avoided Imperial Checkpoints by going through an unmapped portion of the Maw and risked being destroyed by the gravity wells of the blackholes.

Okay, back Star Fighters. When the Imperials lost the Battle of Endor, they didn't collapse. They kept hold to most of their territory. But what happened is they went into an era of Warlords with different Sector Moffs carving out chunks of territory for themselves and waged a civil war aling with the overall Galactic Civil War against the Rebel Alliance. The Warlords killed each other off as well as fall to the New Republic, the survivors formed the Imperial Remnant and stayed as a Galactic Power. They equipped TIE Interceptors with hyperdrives and also developed the TIE Defender, which smoked the T-65 in every way. But it was an expensive bastard to make. It has shieds, hyperdrive, ion cannons, blaster cannons, and a concussion missle and proton torpedoe launcher.

Baron Soontir Fel of the 181st Imperial Fighter Wing used them against the New Republic and later fled with the design to the Chiss Ascendency where they further refined it and used it to great success against the Yuuzhan Vong during their Galactic Invasion.