Modules in Hull Rupture fall into four functional groups, and every module you own is a node in a network rather than an isolated building.
Because modules connect, a production chain that is spread across the ship pays a routing cost for every unit it moves. A tight cluster of production feeding a tight cluster of efficiency feeding the engines will out-perform a larger, scattershot arrangement at the same module count. The efficiency modifiers are what make density worth chasing: doubling the output of a chain you already built is strictly better than adding a second chain, because the second chain costs you the crew to staff it.
The community consensus on defence is consistent: kill zones are the cornerstone of combat. Aliens follow corridors, so they bunch up at natural chokepoints. Find those junctions and put your high-damage turrets there. Barricades stall incoming waves and buy time for the automated defences to finish the job, and pairing barricades with area-of-effect traps maximises the damage done in the seconds they hold.
The other half of the geometry is that you need defence covering multiple breach angles simultaneously. A single chokepoint turret is a trap for you, not for them — if the aliens simply route around it, the chokepoint is a wall you built for yourself. The standard beginner mistake is over-concentrating defence at the obvious entry point.
Repair modules exist because hull damage propagates. When a corridor is breached, the modules behind it stop working, and since those modules are feeding your production chain, a single unrepaired breach takes out a disproportionate slice of your economy. Repair capacity is the difference between a bad wave being an inconvenience and a bad wave ending the run.