The gear in the basic paid-for AmpliTube 4 ("AT4") is
10 Stompbox Models – Acoustic Sim, Chorus, Flanger, Opto Tremolo, Delay, Wah Pedal, Diode Overdrive, Compressor, Graphic EQ, Volume Pedal (can be controlled by external controller)
9 Amplifiers – 5 Classic British Amps: Brit 8000, Brit 9000, Red Pig, Brit Silver and Brit Valve Pre, plus American Tube Clean 1, American Tube Clean 2, British Tube Lead 1, Solid State Bass Preamp
10 Cabinets – 4x12 Brit 8000, 4x12 Brit 9000, 4x12 Red Pig, 4x12 Brit Silver, 4x12 Brit 30, 4x10 Open Vintage, 1x12 Open Vintage, 2x12 Closed Vintage, 4x12 Closed Vintage 1, 1x15 Bass Vintage
29 vintage and modern individual speaker models
4 Microphones – Double Dynamic 57, Dynamic 57, Condenser 414, Condenser 87
2 Rack Effects Units – Digital Delay & Parametric EQ
2 Tuners – UltraTuner plus a Fully chromatic rack tuner
As you say, the others can be bought individually in the Custom Shop or in bundles. Or (I think that) you may qualify for an upgrade/crossgrade to AT4 Deluxe or AT4 Max (have a look at that part of your User Area), if those are suitable for you. Note that each of those has a fixed content (fixed when the bundle was first released) and do not contain all the current gear models. So it is best to do some research before purchasing.