There are different levels of composition. In BIab you can 'composer' a song in about 1 minute or less. It will likey be excellent and you can choose the chords how you wish and the style. You can also go into the depth of the program and tweak.
The other 'end' of this is inputting every note individually and crafting a song from scratch BIAB is not designed for this, you need a sequencer - maybe Real Band (I dont use this). Using Kontakt and East West stuff is primarily designed for the song from scratch approach. This is far more time consuming.
You can also use loops and a combination of all of gthe above methods. Of course to this you can add live recording.

BIAB is primarily set up to use General MIDI. These synths use standard instruments set up in a standard order. These synths come in both hardware and software forms. Kotakt is NOT a GM synth but could emulate one, providing you have the sounds and set the up the right way. Most people use a GM synth for BIAB. BIAB is limited on the classical side.

The Beauty of BIAB is that you have a song right out of the block. You can change its key, adjust its tempo loop copy or alter a few bars, record your guitar played along with it, change the style of the chords from Latin to Heavy Rock or Country. All in a matter of a few minutes work.
There are two knds of tracks MIDI tracks and Real Tracks. Real tracks are REAL musicians. Real tracks are very authenitc sounding you can capture the swinging wails of a harp (mouth organ). Try doing that in MID and you will need skill and a lot of time. PG MIDI tracks are of very high quality too though, so you should not dismiss them.

From BIAB you can export either MIDI or Audio and you can import that into a sequencer and add whatever you want - anything at all.

I use BIAB primariliy as a practice tool. I just fire up a song and play. I have Kotakt and a lot of other synths Play and a lot more besides, but I dont see the need to use them in BIAB. SOme do I suppose. I would use a sequencer for these application - such as Cubase, Logic, Protools. There are also freebies at KVR you could start with, or simply use RealBand.

My strong advice is buy BIAB you will love it and your general musicianship will improve leaps and bounds...

Win 11 64, Asus Rog Strix z390 mobo, 64 gig RAM, 8700k