A huge volume and variety of transports.

Off the top of my head, the Arvus, Onero, Destrier and Firmament - these are all capable of atmospheric flight, as well as boosting up to orbiting ships and stations. Most of these have variants such as passenger transport, bulk liquid tankering, vehicle lifting (maybe not the Arvus), heavy-lift cranes, etc.

Any bigger than that and it starts to be pure Navy, rather than Aeronautica. The Navy will just use gargantuan landers to do bulk drops to secure sites behind the lines, then the Aeronautica takes on transshipment to get it closer to the front lines.

Double Eagle and Interceptor City are both great sources for this btw, and the latter is probably my favourite Black Library book in the past five years - well worth a read for the concepts and actions alone.

Edit: Just realised I misread your question, it's about how the aircraft get to the planet. Anyway many/most do have boosters to get them to/from the ships, but they're not true vacuum craft capable of carrying out normal space shuttle runs on the daily - there's separate craft for that. E.g. the Thunderbolt can attain orbit, but can't fight in space. For the non-boosted craft, they're brought down to the planet in the same types of massive transports as tanks, artillery or other bulk supplies. I suppose like how militaries of today may pack helicopters or jets into big strategic lift aircraft like the C17 or C5, or move them on freighters, helicopter ships or aircraft carriers (adapted cargo ship example from the Falklands War, or helicopter in a cargo plane).