The aliens in Hull Rupture arrive in waves that breach the hull at known times and known positions. That is the single most important thing to understand about the combat layer: you are told when and where an assault comes. You are not told what is in it, and you are not told how many.
Because the assault location is known, the defensive read is always the same shape: find where the breach will happen, decide which corridors the aliens will walk through afterwards, and put your kill zone on that path rather than on the path you would prefer they took. The corridor geometry of your ship is what determines the route, which is why hull choice and layout interact so strongly — the same defence on a different ship is a different defence.
A new assault lands roughly every seven days, and each arrival recalculates your resources, momentum and estimated mission length. This means the defensive threat curve is not a straight ramp. A route that was survivable two nodes ago can become unsurvivable after a single bad encounter, and the "estimated mission length" readout is the closest thing the game has to a difficulty forecast.
The demo's repeatable mode ends in a final boss after twenty waves, and the boss is where the layered-kill-zone approach is tested hardest. Bosses in this game are the reason defensive coverage at multiple angles beats defensive depth at one: a boss that survives your primary chokepoint will walk to your secondary one, and if you never built a secondary one, the wave ends your economy even if it does not end your run.
Individual alien types are the part of Hull Rupture that is least well documented, because the game shipped only days ago and the community wiki landscape was empty at launch. Most public coverage describes enemy behaviour at the wave level rather than unit level. Treat any specific enemy roster you encounter as the ground truth over anything written before launch, and check this page as the community catalogue fills in.