For some that might be Notator, Dorico or other music notation programs. That being one option.
Its all a very personal choice.
I'd venture to say many composers irregardless of their fornal or nonformal background would trigger sounds first from their preferred instruments, developing notation after. Some, myself included play various instruments along while tracking. Be it hardware synths, guitars, drums, bicycle spokes, what-have-you. ; )
It wouldn't be the first time I said its all clay. So I do think asking about "VST's" kind of offsets what answers you might be hoping for.
"What instruments......and your preferred genre(s)..... " would have eeked out some more related responses, me thinks.
As Ari mentioned AU is a common Mac instrumemt/plugin standard and we have a lot of Mac folks here!
Just mentioning this stuff, so in the future it may help. Yeah, its pretty cool to hear what others are doing, have accomplished, as we often only get to hear how they assist others. Its a mixed crowd (wonderfully), and that makes this forum an excellent resource for each of us in one way or another.
Cheers, Jeff!
As for me, I adore Jazz, and really most every genre except hip-hop. I'm also big synth fan from way back so DAW's allowed me to incorporate so much of what I love to do! Make and play music, and get involved in all production aspects. Lately, I'm exploring ambient, but mixing it up with a 70's progressive element. That and cinematic themes.
Everyone here has a cool story.